Enable OPEN_THREAD version of pthread functions on z/OS.
Let's try not add obscure platform-specific #ifdefs like that. pthread_getspecific_d8_np doesn't even return something on Google, so this is pretty obscure as far as this open source project is concerned.
This whole code block is under _LIBCPP_HAS_THREAD_API_PTHREAD, and z/OS doesn't seem to support pthread since it's missing these functions. Instead, z/OS should either provide that API, or you can introduce another "threading back-end" other than the pthread or C11 one.