Attempted locale refactoring. _LIBCPP_LOCALE__L_EXTENSIONS now should be...

Attempted locale refactoring.  _LIBCPP_LOCALE__L_EXTENSIONS now should be defined if one has all of the xxx_l() functions.  I've defined this for apple, freebsd and win32.  _LIBCPP_HAS_DEFAULTRUNELOCALE should be defined if there is a _DefaultRuneLocale.  I've defined this for apple and freebsd.  The block of code we're trying to migrate away from is now under #ifdef __linux__.  I've tested only on OS X.  I hope I haven't broken things too badly elsewhere.  Please let me know.

llvm-svn: 140734