systemc: Make sure sc_start waits for simulation even when starving.

systemc: Make sure sc_start waits for simulation even when starving.

Even if the simulation would return from sc_start immediately because
of starvation, this change ensures that sc_start gives control back
to gem5 so that the scheduler will have a chance to set up
sensitivities, etc., before things get torn down.

Change-Id: I39b1fd704fcbe12c299cad9dbd30258e8fe9d032
Reviewed-on: https://gem5-review.googlesource.com/12218
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>