benchtests: Restore the clock_gettime option

commit 7621e38b
Author: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Date:   Tue Jan 29 17:43:45 2019 +0000

    Add generic hp-timing support

removed the clock_gettime option.  Restore the clock_gettime option for
some x86 CPUs on which value from RDTSC may not be incremented at a fixed
rate.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>