Add _Float128 function aliases.

This patch adds support for *f128 function aliases on platforms where
long double has the binary128 format (and thus GCC 7 provides the
_Float128 type with the same ABI as long double but as a distinct type
in terms of C type compatibility).  This is the same API as provided
in glibc 2.26 for powerpc64le / x86_64 / x86 / ia64 where _Float128
has a different format from long double, with the bulk of the API
coming from TS 18661-3.  All the functions alias the corresponding
long double functions, and __* function names are not provided since
those are only needed once for each floating-point format, not more
than once for different types with the same format (so for example,
-ffinite-math-only maps foof128 to __fool_finite, while type-generic
macros end up calling e.g. __issignalingl for _Float128 arguments on
such platforms).

The preparation for this feature was done in previous patches, so this
one just needs to add the relevant makefile and header definitions,
and update macro definitions of libm_alias_ldouble_other_r, to turn on
the feature, and update documentation and ABI baselines.

Tested (a) for x86_64, (b) for aarch64, (c) with build-many-glibcs.py
with both GCC 6 and GCC 7.

	* sysdeps/ieee754/ldbl-128/Makeconfig: New file.
	* sysdeps/ieee754/ldbl-128/bits/floatn.h: Likewise.
	* sysdeps/ieee754/ldbl-128/float128-abi.h: Likewise.
	* sysdeps/generic/libm-alias-ldouble.h: Include <bits/floatn.h>.
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128]
	(libm_alias_ldouble_other_r): Also create _Float128 alias.
	* sysdeps/ieee754/ldbl-opt/libm-alias-ldouble.h: Include
	<bits/floatn.h>.
	[__HAVE_FLOAT128 && !__HAVE_DISTINCT_FLOAT128]
	(libm_alias_ldouble_other_r): Also create _Float128 alias.
	* manual/math.texi (Mathematics): Document additional architecture
	support for _Float128.
	* sysdeps/unix/sysv/linux/aarch64/libc.abilist: Update.
	* sysdeps/unix/sysv/linux/aarch64/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/alpha/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/alpha/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/libm.abilist: Likewise.

1 files changed, 6 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 464d1ce9b3..ad680db874 100644
--- a/NEWS
+++ b/NEWS
@@ -29,6 +29,12 @@ Major new features:
   in deadlocks and further data corruption.  This change also affects
   process aborts as the result of assertion failures.
 
+* On platforms where long double has the IEEE binary128 format (aarch64,
+  alpha, mips64, s390 and sparc), the math library now implements _Float128
+  interfaces for that type, as defined by ISO/IEC TS 18661-3:2015.  These
+  are the same interfaces added in version 2.26 for some platforms where
+  this format is supported but is not the format of long double.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * On GNU/Linux, the obsolete Linux constant PTRACE_SEIZE_DEVEL is no longer