Revert "[libc++] Workaround timespec_get not always being available in Apple SDKs"

This reverts commit 99f3b231. It breaks
libcxx/modules/stds_include.sh.cpp on macOS as the new include to sys/cdefs.h
causes a dependency from __config to the Darwin module (which already has
a dependency on __config). This cyclic dependency breaks compiling the std
module which breaks compiling pretty much every program with ToT libc++ and
enabled modules.

I'll revert for now to get the bots green again. Sorry for the inconvenience.