[libc++] Remove workarounds for the lack of clock_gettime on older macOS platforms

This increases the Mac OS requirement for building libc++ to 10.12.
Note that it doesn't change whether the *headers* still support older
platforms -- it's only that macOS >= 10.12 is required to build the
dylib from sources.

Differential Revision: https://reviews.llvm.org/D74489