systemc: When sc_start is told to run zero time, do one delta cycle.

systemc: When sc_start is told to run zero time, do one delta cycle.

This is a special case which is mentioned in the spec but hadn't yet
been given any special handling in this implementation.

Change-Id: I500d046f09d916a08e22821f8d3e2f490f8ba5bb
Reviewed-on: https://gem5-review.googlesource.com/12212
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>