systemc: Handle sc_stop called from sc_main correctly.

systemc: Handle sc_stop called from sc_main correctly.

When in sc_main, sc_is_running will return true but we're not going
to run any gem5 events since we're currently in the sc_main Fiber. In
that case, we need to do the sc_stop work inline.

If we're actually running and not just paused, then we do still want to
schedule the work of sc_stop to happen as its own event since that will
happen before returning to sc_main, and actually will likely be the
mechanism by which sc_main starts executing again.

Change-Id: If9ffafc4f240af0f3d9c726b36a0950b5219dc00
Reviewed-on: https://gem5-review.googlesource.com/12269
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>