Provide a C++ version of iszero that does not use __MATH_TG (bug 21930)

When signaling nans are enabled (with -fsignaling-nans), the C++ version
of iszero uses the fpclassify macro, which is defined with __MATH_TG.
However, when support for float128 is available, __MATH_TG uses the
builtin __builtin_types_compatible_p, which is only available in C mode.

This patch refactors the C++ version of iszero so that it uses function
overloading to select between the floating-point types, instead of
relying on fpclassify and __MATH_TG.

Tested for powerpc64le, s390x, x86_64, and with build-many-glibcs.py.

	[BZ #21930]
	* math/math.h [defined __cplusplus && defined __SUPPORT_SNAN__]
	(iszero): New C++ implementation that does not use
	fpclassify/__MATH_TG/__builtin_types_compatible_p, when
	signaling nans are enabled, since __builtin_types_compatible_p
	is a C-only feature.
	* math/test-math-iszero.cc: When __HAVE_DISTINCT_FLOAT128 is
	defined, include ieee754_float128.h for access to the union and
	member ieee854_float128.ieee.
	[__HAVE_DISTINCT_FLOAT128] (do_test): Call check_float128.
	[__HAVE_DISTINCT_FLOAT128] (check_float128): New function.
	* sysdeps/powerpc/powerpc64le/Makefile [subdir == math]
	(CXXFLAGS-test-math-iszero.cc): Add -mfloat128 to the build
	options of test-math-zero on powerpc64le.