libstdc++: Use if consteval rather than if (std::__is_constant_evaluated()) for {,b}float16_t nextafter [PR117321]

The nextafter_c++23.cc testcase fails to link at -O0.
The problem is that eventhough std::__is_constant_evaluated() has
always_inline attribute, that at -O0 just means that we inline the
call, but its result is still assigned to a temporary which is tested
later, nothing at -O0 propagates that false into the if and optimizes
away the if body.  And the __builtin_nextafterf16{,b} calls are meant
to be used solely for constant evaluation, the C libraries don't
define nextafterf16 these days.

As __STDCPP_FLOAT16_T__ and __STDCPP_BFLOAT16_T__ are predefined right
now only by GCC, not by clang which doesn't implement the extended floating
point types paper, and as they are predefined in C++23 and later modes only,
I think we can just use if consteval which is folded already during the FE
and the body isn't included even at -O0.  I've added a feature test for
that just in case clang implements those and implements those in some weird
way.  Note, if (__builtin_is_constant_evaluted()) would work correctly too,
that is also folded to false at gimplification time and the corresponding
if block not emitted at all.  But for -O0 it can't be wrapped into a helper
inline function.

2024-10-29  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/117321
	* include/c_global/cmath (nextafter(_Float16, _Float16)): Use
	if consteval rather than if (std::__is_constant_evaluated()) around
	the __builtin_nextafterf16 call.
	(nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t)): Use
	if consteval rather than if (std::__is_constant_evaluated()) around
	the __builtin_nextafterf16b call.
	* testsuite/26_numerics/headers/cmath/117321.cc: New test.

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