[libc++] Fix clock selection in chrono.cpp and filesystem_clock.cpp

This patch partly reverts the change that was made in 5f1ba3a5
regarding the clock selection on Apple platforms. It turns out that
gettimeofday() is marked as obsolete by POSIX and clock_gettime() is
recommended instead. Since both are equivalent for our purposes,
prefer using clock_gettime() even on Apple platforms.

Differential Revision: https://reviews.llvm.org/D155022