For the C locale we know the encoding is ASCII, so we can avoid using
newlocale and nl_langinfo_l. Similarly, for an unnamed locale, we aren't
going to get a useful answer, so we can just use a default-constrcuted
std::text_encoding representing an unknown encoding.

libstdc++-v3/ChangeLog:

	* src/c++26/text_encoding.cc (__locale_encoding): Add to unnamed
	namespace.
	(std::locale::encoding): Optimize for "C" and "*" names.