[SystemZ][ZOS] Fix the usage of pthread_t within libc++

This is the the minimal change introduced in [[ https://reviews.llvm.org/D88599 | D88599 ]]  to unblock the controversial change and discussion of proper separation between thread from thread id which will continue in D88599.

This patch will address the differences of definition of pthread_t on z/OS vs. Linux and other OS. Main trick to make the code work on z/OS relies on redefining libcpp_thread_id type and _LIBCPP_NULL_THREAD macro. This is necessary to separate initialization of libcxx_thread_id from the one of __libcxx_thread_t;

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D91875