libstdc++: Fix build for targets without FP std::from_chars [PR114633]

If the faster std::from_chars is not supported for floating-point types
then just extract the value from the stream using operator>>.

This fixes a build error for targets where __cpp_lib_to_chars is not
defined.

libstdc++-v3/ChangeLog:

	PR libstdc++/114633
	* include/bits/chrono_io.h (_Parser::operator()) <'S'>: Use
	stream extraction if std::from_chars is not available.