systemc: Ensure the terminated event is notified in all cases.

systemc: Ensure the terminated event is notified in all cases.

The terminated event was being notified if a process was killed, but
not if it was terminated in other ways. This change moves the
notification into the helper which sets termination related state.

Change-Id: I10aa5ad25875db992c8408dc60f087efc76b336b
Reviewed-on: https://gem5-review.googlesource.com/12057
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>