Reorganize locale extension fallbacks. NFCI

The various _l locale extension functions originate from very
different places.  Some come from POSIX, some are BSD extensions,
and some are shared BSD and GLIBC extensions. This patch tries to
group the local extension reimplementations by source. This should
make it easier to make libcxx work with POSIX compliant C libraries
that lack these extensions.

The fallback locale functions are also useful on their own for other
lightweight platforms. Putting these fallback implementations in
support/xlocale should enable code sharing.

I have no access to a newlib system or an android system to build
and test with. I _do_ have access to a system without any of the _l
locale extensions though, and I was able to ensure that the new
__posix_l_fallback.h and __strtonum_fallback.h didn't have any massive
problems.

http://reviews.llvm.org/D17416

llvm-svn: 270213