[libc++] Synchronize clock selection between chrono.cpp and filesystem_clock.cpp

Note that _FilesystemClock will now be implemented by calling gettimeofday()
on Apple platforms instead of clock_gettime(). However, since both are
equivalent, this should not change the behavior on Apple platforms.
There should be no behavior change on other platforms.

In addition to being a consistency clean up, this fixes some issues seen
by folks as reported in https://reviews.llvm.org/D154390#4471924.

Differential Revision: https://reviews.llvm.org/D154457