libstdc++-v3: Implement {,b}float16_t nextafter and some fixes [PR106652]

libstdc++-v3: Implement {,b}float16_t nextafter and some fixes [PR106652]

The following patch implements nextafter for std::{,b}float16_t,
though right now only without constexpr support, and adds a testcase for it.
The testcase unfortunately relevealed I've screwed up testing of my last
patch.  I've tested earlier version of the patch with
--target_board=unix/-std=c++23
but didn't test the final version with that RUNTESTFLAGS, so missed
an invalid call to std::sph_neumann (too many arguments) in the test.
And, I've made a typo in the guard for numeric_limits (the reason
for the guard is I wanted to avoid defining a large macro that nothing
will then use because the std::{,b}float*_t types are C++23 only) and
so numeric_limits wasn't specialized for the types at all but
testsuite/18_support/headers/limits/synopsis_cxx23.cc test didn't
detect that.
In the nextafter implementation I'm calling __builtin_nextafterf
to get various required side-effects for nextafter from 0/-0, or from max
to inf or from min to largest subnormal to avoid needing to set errno
inline, or use inline asm specific for each processor to force math
evaluation barriers.  Dunno if
  #ifdef __INT16_TYPE__
    using __float16_int_type = __INT16_TYPE__;
  #else
    using __float16_int_type = short int;
  #endif
isn't too ugly, perhaps we could just blindly use short int and hope
or even assert it has the same size as _Float16 or __gnu_cxx::__bfloat16_t?
Only aarch64, arm, csky, gcn, x86, nvptx and riscv support these types
and all of them have 16-bit short (I think the only target with some
other short size is avr with certain command line switches where both
short and int are 8-bit, but such mode isn't compatible with C and C++
requirements).

2022-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106652
	* include/std/limits: Fix a typo, 202202L -> 202002L.
	(numeric_limits::<_Float16>::radix, numeric_limits::<_Float32>::radix,
	numeric_limits::<_Float64>::radix, numeric_limits::<_Float128>::radix,
	numeric_limits::<__gnu_cxx::__bfloat16_t>::radix): Use __FLT_RADIX__
	macro instead of type specific macros.
	* include/c_global/cmath (nextafter(_Float16, _Float16)): New
	overload.
	(nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t)):
	Likewise.
	* testsuite/26_numerics/headers/cmath/functions_std_c++23.cc
	(test_functions): Uncomment nextafter test.  Fix up sph_neumann call.
	* testsuite/26_numerics/headers/cmath/nextafter_c++23.cc: New test.