FWCore/Utilities: rdtsc() return 0 on AArc64 (ARMv8)

The functionality is not yet implemented.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Utilities/Timing: rdtscPentium() return 0 on AArch64 (ARMv8)

The functionality not yet implemented.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>