Use gcc attribute ifunc in libc_ifunc macro instead of inline assembly due to false debuginfo.

The current s390 ifunc resolver for vector optimized functions and the common
libc_ifunc macro in include/libc-symbols.h uses something like that to generate ifunc'ed functions:
extern void *__resolve___strlen(unsigned long int dl_hwcap) asm (strlen);
asm (".type strlen, %gnu_indirect_function");

This leads to false debug information:
objdump --dwarf=info libc.so:
...
<1><1e6424>: Abbrev Number: 43 (DW_TAG_subprogram)
    <1e6425>   DW_AT_external    : 1
    <1e6425>   DW_AT_name        : (indirect string, offset: 0x1146e): __resolve___strlen
    <1e6429>   DW_AT_decl_file   : 1
    <1e642a>   DW_AT_decl_line   : 23
    <1e642b>   DW_AT_linkage_name: (indirect string, offset: 0x1147a): strlen
    <1e642f>   DW_AT_prototyped  : 1
    <1e642f>   DW_AT_type        : <0x1e4ccd>
    <1e6433>   DW_AT_low_pc      : 0x998e0
    <1e643b>   DW_AT_high_pc     : 0x16
    <1e6443>   DW_AT_frame_base  : 1 byte block: 9c     (DW_OP_call_frame_cfa)
    <1e6445>   DW_AT_GNU_all_call_sites: 1
    <1e6445>   DW_AT_sibling     : <0x1e6459>
 <2><1e6449>: Abbrev Number: 44 (DW_TAG_formal_parameter)
    <1e644a>   DW_AT_name        : (indirect string, offset: 0x1845): dl_hwcap
    <1e644e>   DW_AT_decl_file   : 1
    <1e644f>   DW_AT_decl_line   : 23
    <1e6450>   DW_AT_type        : <0x1e4c8d>
    <1e6454>   DW_AT_location    : 0x122115 (location list)
...

The debuginfo for the ifunc-resolver function contains the DW_AT_linkage_name
field, which names the real function name "strlen". If you perform an inferior
function call to strlen in lldb, then it fails due to something like that:
"error: no matching function for call to 'strlen'
candidate function not viable: no known conversion from 'const char [6]'
to 'unsigned long' for 1st argument"

The unsigned long is the dl_hwcap argument of the resolver function.
The strlen function itself has no debufinfo.

The s390 ifunc resolver for memset & co uses something like that:
asm (".globl FUNC"
     ".type FUNC, @gnu_indirect_function"
     ".set FUNC, __resolve_FUNC");

This way the debuginfo for the ifunc-resolver function does not conain the
DW_AT_linkage_name field and the real function has no debuginfo, too.

Using this strategy for the vector optimized functions leads to some troubles
for functions like strnlen. Here we have __strnlen and a weak alias strnlen.
The __strnlen function is the ifunc function, which is realized with the asm-
statement above. The weak_alias-macro can't be used here due to undefined symbol:
gcc ../sysdeps/s390/multiarch/strnlen.c -c ...
In file included from <command-line>:0:0:
../sysdeps/s390/multiarch/strnlen.c:28:24: error: ‘strnlen’ aliased to undefined symbol ‘__strnlen’
 weak_alias (__strnlen, strnlen)
                        ^
./../include/libc-symbols.h:111:26: note: in definition of macro ‘_weak_alias’
   extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));
                          ^
../sysdeps/s390/multiarch/strnlen.c:28:1: note: in expansion of macro ‘weak_alias’
 weak_alias (__strnlen, strnlen)
 ^
make[2]: *** [build/string/strnlen.o] Error 1

As the __strnlen function is defined with asm-statements the function name
__strnlen isn't known by gcc. But the weak alias can also be done with an
asm statement to resolve this issue:
__asm__ (".weak  strnlen\n\t"
         ".set   strnlen,__strnlen\n");

In order to use the weak_alias macro, gcc needs to know the ifunc function. The
minimum gcc to build glibc is currently 4.7, which supports attribute((ifunc)).
See https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html.
It is only supported if gcc is configured with --enable-gnu-indirect-function
or gcc supports it by default for at least intel and s390x architecture.
This patch uses the old behaviour if gcc support is not available.
Usage of attribute ifunc is something like that:
__typeof (FUNC) FUNC __attribute__ ((ifunc ("__resolve_FUNC")));

Then gcc produces the same .globl, .type, .set assembler instructions like above.
And the debuginfo does not contain the DW_AT_linkage_name field and there is no
debuginfo for the real function, too.

But in order to get it work, there is also some extra work to do.
Currently, the glibc internal symbol on s390x e.g. __GI___strnlen is not the
ifunc symbol, but the fallback __strnlen_c symbol. Thus I have to omit the
libc_hidden_def macro in strnlen.c (here is the ifunc function __strnlen)
because it is already handled in strnlen-c.c (here is __strnlen_c).

Due to libc_hidden_proto (__strnlen) in string.h, compiling fails:
gcc ../sysdeps/s390/multiarch/strnlen.c -c ...
In file included from <command-line>:0:0:
../sysdeps/s390/multiarch/strnlen.c:53:24: error: ‘strnlen’ aliased to undefined symbol ‘__strnlen’
 weak_alias (__strnlen, strnlen)
                        ^
./../include/libc-symbols.h:111:26: note: in definition of macro ‘_weak_alias’
   extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));
                          ^
../sysdeps/s390/multiarch/strnlen.c:53:1: note: in expansion of macro ‘weak_alias’
 weak_alias (__strnlen, strnlen)
 ^
make[2]: *** [build/string/strnlen.os] Error 1

I have to redirect the prototypes for __strnlen in string.h and create a copy
of the prototype for using as ifunc function:
__typeof (__redirect___strnlen) __strnlen __attribute__ ((ifunc ("__resolve_strnlen")));
weak_alias (__strnlen, strnlen)

This way there is no trouble with the internal __GI_* symbols.
Glibc builds fine with this construct and the debuginfo is "correct".
For functions without a __GI_* symbol like memccpy this redirection is not needed.

This patch adjusts the common libc_ifunc and libm_ifunc macro to use gcc
attribute ifunc. Due to this change, the macro users where the __GI_* symbol
does not target the ifunc symbol have to be prepared with the redirection
construct.
Furthermore a configure check to test gcc support is added. If it is not supported,
the old behaviour is used.

This patch also prepares the libc_ifunc macro to be useable in s390-ifunc-macro.
The s390 ifunc-resolver-functions do have an hwcaps parameter and not all
resolvers need the same initialization code. The next patch in this series
changes the s390 ifunc macros to use this common one.

ChangeLog:

	* include/libc-symbols.h (__ifunc_resolver):
	New macro is used by __ifunc* macros.
	(__ifunc): New macro uses gcc attribute ifunc or inline assembly
	depending on HAVE_GCC_IFUNC.
	(libc_ifunc, libm_ifunc): Use __ifunc as base macro.
	(libc_ifunc_redirected, libc_ifunc_hidden, libm_ifunc_init): New macro.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_finite.c:
	Redirect ifunced function in header for using as type for ifunc function.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_finitef.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isinf.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isinff.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isnan.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/memcmp.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/memcpy.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/memmove.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/mempcpy.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/memset.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/rawmemchr.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/strchr.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/strlen.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/strncmp.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/multiarch/strnlen.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_finite.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_finitef.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isinf.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isinff.c: Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnan.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memcmp.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/mempcpy.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/rawmemchr.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpncpy.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcat.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strchr.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcmp.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcpy.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncpy.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strnlen.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strrchr.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strstr.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/wcschr.c: Likewise.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isnanf.c:
	Add libc_hidden_def() and use libc_ifunc_hidden() macro
	instead of libc_ifunc() macro.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnanf.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpcpy.c: Likewise.

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