libstdc++: Rework std::format support for wchar_t

This changes how std::format creates wide strings, by replacing uses of
std::ctype<wchar_t>::widen with the recently-added __to_wstring_numeric
helper function. This removes the dependency on the locale, which should
only be used for locale-specific formats such as {:Ld}.

Also disable all the wide string formatting support if the
_GLIBCXX_USE_WCHAR_T macro is not defined. This is consistent with other
wchar_t support being disabled if the library is built without that
macro defined.

libstdc++-v3/ChangeLog:

	* include/std/format [_GLIBCXX_USE_WCHAR_T]: Guard all wide
	string formatters with this macro.
	(__formatter_int::_M_format_int, __formatter_fp::format)
	(formatter<const void*, C>::format): Use __to_wstring_numeric
	instead of std::ctype::widen.
	(__formatter_fp::_M_localize): Use hardcoded wchar_t values
	instead of std::ctype::widen.
	* testsuite/std/format/functions/format.cc: Add more checks for
	wstring formatting of arithmetic types.