[libc++] Check _LIBCPP_USE_CLOCK_GETTIME before using clock_gettime

The clock_gettime function is available when _POSIX_TIMERS is defined.
We check for this and set _LIBCPP_USE_CLOCK_GETTIME accordingly since
59b31027. But check for _LIBCPP_USE_CLOCK_GETTIME was removed in
babd3aef. As a result, code is now trying to use clock_gettime even
on platforms where it is not available and it is causing build failure
with newlib.

This patch restores the checks to fix this.

Differential Revision: https://reviews.llvm.org/D88825