systemc: Ignore direct requests to mark a process as ready.

systemc: Ignore direct requests to mark a process as ready.

That might happen when a process is being marked as ready at the start
of simulation.

Because the process might not end up on the ready list, displacing it
from the init list, excplicitly pop it off the init list as well.

Change-Id: Iebf972e3e1baedec17b9b99b4da9dd44cd8e6957
Reviewed-on: https://gem5-review.googlesource.com/12047
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>