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 _Float128 overload in terms of a
non-existing from_chars_strtod function.

Only try to use __strtof128 if uselocale is available and therefore we
can use the from_chars_strtod function template.

This is a simpler change than r14-1431-g7037e7b6e4ac41 on trunk, because
that caused unwanted ABI regressions (PR libstdc++/110077).

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.