libstdc++: Define std::numeric_limits<_FloatNN> before C++23

The extended floating-point types such as _Float32 are supported by GCC
prior to C++23, you just can't use the standard-conforming names from
<stdfloat> to refer to them. This change defines the specializations of
std::numeric_limits for those types for older dialects, not only for
C++23.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (__gnu_cxx::__bfloat16_t): Define
	whenever __BFLT16_DIG__ is defined, not only for C++23.
	* include/std/limits (numeric_limits<bfloat16_t>): Likewise.
	(numeric_limits<_Float16>, numeric_limits<_Float32>)
	(numeric_limits<_Float64>): Likewise for other extended
	floating-point types.