libstdc++: Use fast_float in std::from_chars for binary32/64
This makes our std::from_chars implementation use fast_float for decimal
parsing of binary32/64 numbers. For other floating-point formats we
still use the fallback implementation that goes through the strtod family
of functions.
libstdc++-v3/ChangeLog:
* src/c++17/floating_from_chars.cc: (USE_LIB_FAST_FLOAT):
Conditionally define, and use it to conditionally include
fast_float.
(from_chars): Use fast_float for float and double when
USE_LIB_FAST_FLOAT.