[ARM] Convert suspend to a state machine.

[ARM] Convert suspend to a state machine.

The original version had issues when two suspend events came
in at around the same time, causing APM to get confused:
threads became stuck in APM_IOC_SUSPEND and suspends_pending
incremented on each apm --suspend call.

Now, we only add a suspend event to a users queue and increment
suspends_pending if the user isn't already in the middle of
handling a suspend event.