We used to store the wall-to-monotonic offset and the realtime base.
It's faster to precompute the monotonic base.

This is about a 3% speedup on Sandy Bridge for CLOCK_MONOTONIC.
It's much more impressive for CLOCK_MONOTONIC_COARSE.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>