Make locale code compile on CloudABI.

After r241454 landed, libc++'s locale code compiles on CloudABI, with
the exception of the following two bits:

- CloudABI doesn't have setlocale(), as the C library does not keep
  track of any global state. The global locale is always set to "C".
  Disable the call to setlocale() on this system.
- Similarly, mbtowc_l() is also not present, as it is also not
  thread-safe. As CloudABI does not support state-dependent encodings,
  simply disable that part of the logic.

The locale code now compiles out of the box on CloudABI.

Differential Revision:	http://reviews.llvm.org/D10729
Reviewed by:	jroelofs

llvm-svn: 241455