libstdc++: Fix std::format for chrono::duration with unsigned rep [PR115668]

libstdc++: Fix std::format for chrono::duration with unsigned rep [PR115668]

Using std::chrono::abs is only valid if numeric_limits<rep>::is_signed
is true, so using it unconditionally made it ill-formed to format a
duration with an unsigned rep.

The duration formatter might as negate the duration itself instead of
using chrono::abs, because it already needs to check for a negative
value.

libstdc++-v3/ChangeLog:

	PR libstdc++/115668
	* include/bits/chrono_io.h (formatter<duration<R,P, C>::format):
	Do not use chrono::abs.
	* testsuite/20_util/duration/io.cc: Check formatting a duration
	with unsigned rep.