This adds the ClockNanosleep syscall and the TIMER_* constants;
currently only TIMER_ABSTIME for specifying an absolute timestamp.

Change-Id: Ic8d857eb92818529aadab569cd8646a19bb9c055
Reviewed-on: https://go-review.googlesource.com/c/157658
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>