systemc: Mostly implememt sc_simcontext.

systemc: Mostly implememt sc_simcontext.

Since we don't track the current process using the sc_curr_proc_handle
structure, we keep one around just to return from the appropriate
accessor, and set its values when it's requested. If the object is
kept around, those values won't change to track changing processes.

From what I see, none of the tests rely on the value tracking the
process beyond the callsight.

Change-Id: I1ad3b7a7b15aa0bc4d218f986ffbe7c51501b296
Reviewed-on: https://gem5-review.googlesource.com/c/12611
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>