libstdc++: Make std::println use locale from ostream (LWG 4088)

This was just approved in Wrocław.

libstdc++-v3/ChangeLog:

	* include/std/ostream (println): Pass stream's locale to
	std::format, as per LWG 4088.
	* testsuite/27_io/basic_ostream/print/1.cc: Check std::println
	with custom locale. Remove unused brit_punc class.