Use CLOCK_MONOTONIC_COARSE for the fast time source, except on POWER/PowerPC Linux, where CLOCK_MONOTONIC is faster than CLOCK_MONOTONIC_COARSE.
Use CLOCK_MONOTONIC_COARSE for the fast time source, except on POWER/PowerPC
Linux, where CLOCK_MONOTONIC is faster than CLOCK_MONOTONIC_COARSE.

Also remove use of gettimeofday(2) in that case for now.

mercury-hpc/mercury,mercury-hpc/mercury,mercury-hpc/mercury,mercury-hpc/mercury