systemc: Handle event notifications scheduled before sc_start.

systemc: Handle event notifications scheduled before sc_start.

After sc_start is called, gem5 has run far enough to have an event
queue to schedule the notification events on. Before then, it's still
legal to request a timed notification. The scheduler should keep track
of those requests, and once an event queue is available it should
add them to it.

Change-Id: Ie7445b1f2e616f4bd36044a09dbef9e1d12d7350
Reviewed-on: https://gem5-review.googlesource.com/12036
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>