systemc: Separate and conditionalize exposing sc_main to python.

Change-Id: Ib39dd79c607b277ba94f90dee41c09c1b3b66481
Reviewed-on: https://gem5-review.googlesource.com/c/13978
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
3 files changed