systemc: Implement sc_get_current_process_handle's other personality.

systemc: Implement sc_get_current_process_handle's other personality.

When run during simulation, sc_get_current_process_handle returns a
handle for the currently running process or a invalid handle if no
process is running (ie sc_main is running).

When run during elaboration, it returns a handle to the most recently
created process. This second context is what this change handles.

Change-Id: I3fb247b9b7bf83891c782966cfef474753159158
Reviewed-on: https://gem5-review.googlesource.com/12030
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>