systemc: When sc_start-ing with zero time, ensure the ready event runs.

systemc: When sc_start-ing with zero time, ensure the ready event runs.

The ready event is what notices that we only wanted to run one delta
cycle, or no delta cycle if there was nothing to do, and return to
sc_main. If the ready event wasn't scheduled, we would advance time
before the ready event ran and returned to sc_main which is incorrect.

Change-Id: Ic3c10a2f1405f744e8c2bd37aa45846ee6e98e12
Reviewed-on: https://gem5-review.googlesource.com/12267
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>