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.