powerpc/vdso64: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE
Current vDSO64 implementation does not have support for coarse clocks
(CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME_COARSE), for which it falls back
to system call, increasing the response time, vDSO implementation reduces
the cycle time. Below is a benchmark of the difference in execution times.
(Non-coarse clocks are also included just for completion)