systemc: Change how sc_clock creates processes to match the tests.

systemc: Change how sc_clock creates processes to match the tests.

Accellera sets up the mechanism which toggles sc_clock differently
than it's set up in gem5. This change moves things around a little to
more closely match the order things are done by Accellera so that the
test output matches.

Change-Id: Ia6d327f4cd5d689f6969398f02a66278a3dc010c
Reviewed-on: https://gem5-review.googlesource.com/c/12609
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>