Fix wakeup latency when sleeping with 'wait'

Fix wakeup latency when sleeping with 'wait'

If we handle an interrupt just before the 'wait' and the interrupt
schedules some work, we need to skip the 'wait' call. The simple solution
of calling sched_runnable() with interrupts disabled immediately before
wait still leaves a window after the call and before 'wait' in which
the same issue can occur.

The solution implemented is to check the EPC in the interrupt handler, and
if it is in a region before the 'wait' call, to fix up the EPC to skip the
wait call.

Reported/analysed by:	adrian
Fix suggested by:	kib

Reviewed by:	jmallett, imp