libstdc++: Fix std::format thousands separators when sign present [PR120548]

libstdc++: Fix std::format thousands separators when sign present [PR120548]

The leading sign character should be skipped when deciding whether to
insert thousands separators into a floating-point format.

libstdc++-v3/ChangeLog:

	PR libstdc++/120548
	* include/std/format (__formatter_fp::_M_localize): Do not
	include a leading sign character in the string to be grouped.
	* testsuite/std/format/functions/format.cc: Check grouping when
	sign is present in the output.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>