time: Expose getrawmonotonic64 for in-kernel uses

Adds a timespec64 based getrawmonotonic64() implementation
that can be used as we convert internal users of
getrawmonotonic away from using timespecs.

Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --cc include/linux/timekeeping.h
Simple merge

diff --cc kernel/time/timekeeping.c
Simple merge