[libc++] Port the time functions to z/OS

This patch adds a shim for missing time functions on z/OS, and adds a
layer of indirection to account for differences in the timespec struct
on different systems.

This was originally committed as 173b5116 and reverted in 777ca48c
because the original commit also checked-in unrelated changes.

Differential Revision: https://reviews.llvm.org/D87940