runtime: use clock_gettime instead of gettimeofday on Darwin

clock_gettime has higher resolution, we should use it if available.
It is not guaranteed to be available until Darwin 10.12.  This issue is on hold until that is our minimum Darwin version.

We could do this now but it would require making the choice at runtime.  That seems difficult - we'd need to use the Darwin equivalent of dlopen/dlsym to query the existence of, and call, clock_gettime.
