libstdc++: Fix preprocessor conditions for std::from_chars [PR109921]

libstdc++: Fix preprocessor conditions for std::from_chars [PR109921]

We use the from_chars_strtod function with __strtof128 to read a
_Float128 value, but from_chars_strtod is not defined unless uselocale
is available. This can lead to compilation failures for some targets,
because we try to define the _Flaot128 overload in terms of a
non-existing from_chars_strtod function.

Only try to use __strtof128 if uselocale is available, otherwise
fallback to the long double overload of std::from_chars (which might
fallback to the double overload, which should use fast_float).

This ensures we always define the full set of overloads, even if they
are not always accurate for all values of the wider types.

libstdc++-v3/ChangeLog:

	PR libstdc++/109921
	* src/c++17/floating_from_chars.cc (USE_STRTOF128_FOR_FROM_CHARS):
	Only define when USE_STRTOD_FOR_FROM_CHARS is also defined.
	(USE_STRTOD_FOR_FROM_CHARS): Do not undefine when long double is
	binary64.
	(from_chars(const char*, const char*, double&, chars_format)):
	Check __LDBL_MANT_DIG__ == __DBL_MANT_DIG__ here.
	(from_chars(const char*, const char*, _Float128&, chars_format))
	Only use from_chars_strtod when USE_STRTOD_FOR_FROM_CHARS is
	defined, otherwise parse a long double and convert to _Float128.