[SystemZ][z/OS] Enable POSIX_l functions for z/OS

The aim of this patch is to enable POSIX _l functions for z/OS. In particular, the functions are provided with libc++ and this patch resorts to the fallback functions. Nonetheless, the functions are being added so the implementation of the ctype<> member functions can call them. The following changes were needed to allow for a successful build when using the libc++ library for z/OS.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D90319