systemc: Keep track of more cases when we should be ready after resume.

systemc: Keep track of more cases when we should be ready after resume.

If a thread self suspends, it should be marked as ready after resuming.
If a process was already ready when suspended, it should also be
remarked as ready after resuming.

Special care has to be taken in pre-initialization situations so that
processes are put on the right lists, and whether a process is tracked
is already marked as ready.

Change-Id: I15da7d747db591785358d47781297468c5f9fd09
Reviewed-on: https://gem5-review.googlesource.com/c/12445
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>