Ensure alarms are called back when they expire in the past

Turns out the posix timers we're dealing with aren't well behaved.

If the timer is TIMER_ABSTIME the following is supposed to be true:
"If the specified time has already passed, the function shall succeed
and the expiration notification shall be made."

But alas, this is not the case. If the expiration time happens to be
in the past (e.g. very short timer which gets hit with a context switch
before the timer can be set) the timer decides to disarm itself. This
means no more callbacks happen, and no more alarms are processed ever.

sadness.

But thankfully, we can use timer_gettime to check the state of the timer
after timer_settime. If timer_gettime tells us the timer is disarmed
right after we armed it, we want to perform the alarm callback
ourselves.

Put all timer callbacks on the same thread (as would be the case in the
underlying timer implementation already) and used a sempahore to signal
when an alarm expires in the normal posix timer callback and also in the
timer didn't set case.

Change-Id: I39854b241369b2da52afbaaba0c01d3167a47e32