Cleanup: prefer _LIBCPP_GET_C_LOCALE over __cloc().

The __cloc() function is only present in case the environment does not
provide a way to refer to the C locale using a compile-time constant
expression. _LIBCPP_GET_C_LOCALE seems to be defined unconditionally.

This improves compilation of the locale code on CloudABI.

Differential Revision:	http://reviews.llvm.org/D10690
Reviewed by:	jroelofs

llvm-svn: 241454