libstdc++: Fix libstdc++ build on some targets [PR107811]

libstdc++: Fix libstdc++ build on some targets [PR107811]

fast_float library relies on size_t being 32-bit or larger and float/double
being IEEE single/double.  Otherwise we only use strtod/strtof.
In 3 spots I've used fast_float namespace stuff unconditionally in one
function, which breaks the build if fast_float is disabled.

2022-11-23  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107811
	* src/c++17/floating_from_chars.cc (__floating_from_chars_hex): Guard
	fast_float uses with #if USE_LIB_FAST_FLOAT and for mantissa_bits and
	exponent_bits provide a fallback.