libstdc++: Small extended float support tweaks

The following patch
1) enables the std::float128_t overloads for x86 with glibc 2.26+
2) makes std::nextafter(std::float16_t, std::float16_t) and
   std::nextafter(std::bfloat16_t, std::bfloat16_t) constexpr
3) adds (small) testsuite coverage for that

2022-10-21  Jakub Jelinek  <jakub@redhat.com>

	* config/os/gnu-linux/os_defines.h (_GLIBCXX_HAVE_FLOAT128_MATH):
	Uncomment.
	* include/c_global/cmath (nextafter(_Float16, _Float16)): Make it constexpr.
	If std::__is_constant_evaluated() call __builtin_nextafterf16.
	(nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t)): Similarly
	but call __builtin_nextafterf16b.
	* testsuite/26_numerics/headers/cmath/nextafter_c++23.cc (test): Add
	static assertions to test constexpr nextafter.