systemc: If an event is a delta notification, checked if it's timed.

systemc: If an event is a delta notification, checked if it's timed.

If we're descheduling an event which is at the current time, it may
have been scheduled as a delta notification, but it could have also
been scheduled as a timed notification and we just got to that point
in time. If an event is for the current time but isn't in the delta
notifications, this change detects that and then treats it as a timed
notification.

Change-Id: I1d8f4c40325cc7f355b7f2e6f08611483ce11858
Reviewed-on: https://gem5-review.googlesource.com/12250
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>