libstdc++: Revert pre-C++23 support for 16-bit float types [PR111060]

libstdc++: Revert pre-C++23 support for 16-bit float types [PR111060]

In r14-3304-g1a566fddea212a and r14-3305-g6cf214b4fc97f5 I tried to
enable std::format for 16-bit float types before C++23. This causes
errors for targets where the types are defined but can't actually be
used, e.g. i686 without sse2.

Make the std::numeric_limits and std::formatter specializations for
_Float16 and __bfloat16_t depend on the __STDCPP_FLOAT16_T__ and
__STDCPP_BFLOAT16_T__ macros again, so they're only defined for C++23
when the type is fully supported. This is OK because the main point of
my earlier commits was to add better support for _Float32 and _Float64.
It seems fine for the new 16-bit types to only be supported for C++23,
as they were never present before GCC 13 anyway.

libstdc++-v3/ChangeLog:

	PR target/111060
	* include/std/format (formatter): Only define specializations
	for 16-bit floating-point types for C++23.
	* include/std/limits (numeric_limits): Likewise.