Revert "configure: Don't define HAVE_CLOCK_GETTIME on macOS.". All our supported macOS versions (10.12 and newer) now have clock_gettime(). This reverts commit 5ddcfa019d027d9d690c98151c708eb4e7d5f72b.
ntdll: Always use mach_continuous_time() on macOS. mach_continuous_time() is available on all our supported macOS versions (10.12 and newer).