libstdc++: Do not apply localized formatting to NaN and inf [PR114863]

We don't want to add grouping to strings like "-inf", and there is no
radix character to replace either.

libstdc++-v3/ChangeLog:

	PR libstdc++/114863
	* include/std/format (__formatter_fp::format): Only use
	_M_localized for finite values.
	* testsuite/std/format/functions/format.cc: Check localized
	formatting of NaN and initiny.