libstdc++: Use std::from_chars to speed up parsing subsecond durations

With std::from_chars we can parse subsecond durations much faster than
with std::num_get, as shown in the microbenchmarks below. We were using
std::num_get and std::numpunct in order to parse a number with the
locale's decimal point character. But we copy the chars from the input
stream into a new buffer anyway, so we can replace the locale's decimal
point with '.' in that buffer, and then we can use std::from_chars on
it.

Benchmark                Time             CPU   Iterations
----------------------------------------------------------
from_chars_millisec    158 ns          158 ns      4524046
num_get_millisec       192 ns          192 ns      3644626
from_chars_microsec    164 ns          163 ns      4330627
num_get_microsec       205 ns          205 ns      3413452
from_chars_nanosec     173 ns          173 ns      4072653
num_get_nanosec        227 ns          227 ns      3105161

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (_Parser::operator()): Use
	std::from_chars to parse fractional seconds.