posix-timers: Add timer_rearm() callback

Add a timer_rearm() callback which is used to make the rescheduling of
posix interval timers independent of the underlying clock implementation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211656.732632167@linutronix.de