[PATCH] [libc++] Support newlib as libc++'s C library [locale part]

[Marshall Clow]

A general comment:  How much of this file is the same as `$LIBCXX/include/support/bionic/bionic_locale.h`, and would it be beneficial/desirable to create a file that stubs out the locale routines and share it between those two implementations.

Other than that, this looks pretty good.
I like the lack of damage to `<locale>` a lot.

http://reviews.llvm.org/D5385