libstdc++: std::from_chars std::{,b}float16_t support

The following patch adds std::from_chars support, similarly to the
previous std::to_chars patch through APIs that use float instead of
the 16-bit floating point formats as container.
The patch uses the fast_float library and doesn't need any changes
to it, like the previous patch it introduces wrapper classes around
float that represent the float holding float16_t or bfloat16_t value,
and specializes binary_format etc. from fast_float for these classes.

The new test verifies exhaustively to_chars and from_chars afterward
results in the original value (except for nans) in all the fmt cases.

2022-11-01  Jakub Jelinek  <jakub@redhat.com>

	* include/std/charconv (__from_chars_float16_t,
	__from_chars_bfloat16_t): Declare.
	(from_chars): Add _Float16 and __gnu_cxx::__bfloat16_t overloads.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.31): Export
	_ZSt22__from_chars_float16_tPKcS0_RfSt12chars_format and
	_ZSt23__from_chars_bfloat16_tPKcS0_RfSt12chars_format.
	* src/c++17/floating_from_chars.cc
	(fast_float::floating_type_float16_t,
	fast_float::floating_type_bfloat16_t): New classes.
	(fast_float::binary_format<floating_type_float16_t>,
	fast_float::binary_format<floating_type_bfloat16_t>): New
	specializations.
	(fast_float::to_float<floating_type_float16_t>,
	fast_float::to_float<floating_type_bfloat16_t>,
	fast_float::to_extended<floating_type_float16_t>,
	fast_float::to_extended<floating_type_bfloat16_t>): Likewise.
	(fast_float::from_chars_16): New template function.
	(__floating_from_chars_hex): Allow instantiation with
	fast_float::floating_type_{,b}float16_t.
	(from_chars): Formatting fixes for float/double/long double overloads.
	(__from_chars_float16_t, __from_chars_bfloat16_t): New functions.
	* testsuite/20_util/to_chars/float16_c++23.cc: New test.

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