libstdc++: Add pretty printer for std::locale

libstdc++: Add pretty printer for std::locale

Print the locale's name, except when it uses the same named C locale for
all categories except one, in which case print something like:
std::locale = "en_GB.UTF-8" with "LC_CTYPE=en_US.UTF-8"

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdLocalePrinter): New
	printer class.
	* testsuite/libstdc++-prettyprinters/locale.cc: New test.