systemc: Add some missing functions which interact with the scheduler.

Change-Id: Ifc8c8d4a7bb6e941485e80f4884cfa4bb648c17c
Reviewed-on: https://gem5-review.googlesource.com/10846
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>