[libc] Add implemementations of thread specific data related API.

Specifically, POSIX functions pthread_key_create, pthread_key_delete,
pthread_setspecific and pthread_getspecific have been added. The C
standard equivalents tss_create, tss_delete, tss_set and tss_get have
also been added.

Reviewed By: lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D131647