systemc: When stopping immediately, block new processes/updates.

systemc: When stopping immediately, block new processes/updates.

When stopping immediately, we're supposed to finish the current
process but not run any other processes or go to the update phase. The
rest of the process could introduce new processes or request new
updates, so we need to make sure we block those if we're in the process
of stopping.

Change-Id: I9cc867d294cf171dfedb4b9d43fbc167c2057de8
Reviewed-on: https://gem5-review.googlesource.com/c/12466
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>