[SystemZ][ZOS] Provide PATH_MAX macro for libcxx

Defining PATH_MAX to _XOPEN_PATH_MAX which is the closest macro available on z/OS.
Note that this value is 1024 which is 4 times smaller from same macro on Linux.

Reviewed By: #libc, ldionne, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D92110