This is one part of many of a locale refactor. See http://reviews.llvm.org/D17146 for an idea of where this is going.
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.