systemc: Clear out the scheduler more agressively and with common code.

systemc: Clear out the scheduler more agressively and with common code.

It's be useful/necessary to flush pending activity even when not
tearing down the scheduler, specifically when stopping.

Change-Id: I6b3716a8ca1f8ca151222e08f30bd3c9a43364b9
Reviewed-on: https://gem5-review.googlesource.com/12248
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>