systemc: Make sc_(pause|stop) exit to python when not using sc_main.

systemc: Make sc_(pause|stop) exit to python when not using sc_main.

In those cases, there's no sc_main to return control to. The python
config script is serving more or less the same purpose, so we can
return control to there instead.

Change-Id: I3cf0623ae51d989b883fb8556ebbf44651bbec99
Reviewed-on: https://gem5-review.googlesource.com/c/16445
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>