Add an ignore_status_change parameter to OS_pause for SMP-IDLE.

Add an ignore_status_change parameter to OS_pause for SMP-IDLE.

OS_pause previously returned immediately if there were pending
interrupts OR subprocess status changes, as required by a blocking
test-select-registry primitive.  That made it useless to the SMP-IDLE
primitive, which spins in the idle loop until the io-waiter (or a
timer interrupt handler) gets around to syncing subprocess statuses.
It will spin too after getting a SIGCHLD until statuses are synced.