Add iscanonical.

TS 18661-1 adds an iscanonical classification macro to <math.h>.

The motivation for this is decimal floating-point, where some values
have both canonical and noncanonical encodings.  For IEEE binary
interchange formats, all encodings are canonical.  For x86/m68k
ldbl-96, and for ldbl-128ibm, there are encodings that do not
represent any valid value of the type; although formally iscanonical
does not need to handle trap representations (and so could just always
return 1), it seems useful, and in line with the description in the TS
of "representations that are extraneous to the floating-point model"
as being non-canonical (as well as "redundant representations of some
or all of its values"), for it to detect those representations and
return 0 for them.

This patch adds iscanonical to glibc.  It goes in a header
<bits/iscanonical.h>, included under appropriate conditions in
<math.h>.  The default header version just evaluates the argument
(converted to its semantic type, though current GCC will probably
discard that conversion and any exceptions resulting from it) and
returns 1.  ldbl-96 and ldbl-128ibm then have versions of the header
that call a function __iscanonicall for long double (the sizeof-based
tests will of course need updating for float128 support, like other
such type-generic macro implementations).  The ldbl-96 version of
__iscanonicall has appropriate conditionals to reflect the differences
in the m68k version of that format (where the high mantissa bit may be
either 0 or 1 when the exponent is 0 or 0x7fff).  Corresponding tests
for those formats are added as well.  Other architectures do not have
any new functions added because just returning 1 is correct for all
their floating-point formats.

Tested for x86_64, x86, mips64 (to test the default macro version) and
powerpc.

	* math/math.h [__GLIBC_USE (IEC_60559_BFP_EXT)]: Include
	<bits/iscanonical.h>.
	* bits/iscanonical.h: New file.
	* math/s_iscanonicall.c: Likewise.
	* math/Versions (__iscanonicall): New libm symbol at version
	GLIBC_2.25.
	* math/libm-test.inc (iscanonical_test_data): New array.
	(iscanonical_test): New function.
	(main): Call iscanonical_test.
	* math/Makefile (headers): Add bits/iscanonical.h.
	(type-ldouble-routines): Add s_iscanonicall.
	* manual/arith.texi (Floating Point Classes): Document
	iscanonical.
	* manual/libm-err-tab.pl: Update comment on interfaces without
	ulps tabulated.
	* sysdeps/ieee754/ldbl-128ibm/bits/iscanonical.h: New file.
	* sysdeps/ieee754/ldbl-128ibm/s_iscanonicall.c: Likewise.
	* sysdeps/ieee754/ldbl-128ibm/test-iscanonical-ldbl-128ibm.c:
	Likewise.
	* sysdeps/ieee754/ldbl-128ibm/Makefile (tests): Add
	test-iscanonical-ldbl-128ibm.
	* sysdeps/ieee754/ldbl-96/bits/iscanonical.h: New file.
	* sysdeps/ieee754/ldbl-96/s_iscanonicall.c: Likewise.
	* sysdeps/ieee754/ldbl-96/test-iscanonical-ldbl-96.c: Likewise.
	* sysdeps/ieee754/ldbl-96/Makefile: Likewise.
	* sysdeps/unix/sysv/linux/i386/libm.abilist: Update.
	* sysdeps/unix/sysv/linux/ia64/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/m68k/m680x0/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libm.abilist:
	Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libm.abilist:
	Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libm-le.abilist:
	Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libm.abilist:
	Likewise.
	* sysdeps/unix/sysv/linux/x86_64/64/libm.abilist: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/x32/libm.abilist: Likewise.

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