systemc: Set the scheduler's event queue earlier.

systemc: Set the scheduler's event queue earlier.

The kernel can set the event queue during its own construction which
will ensure that the scheduler can schedule events as early as
possible.

Change-Id: I0e47ca0a667e77d36c97860cd7c6b7577415c801
Reviewed-on: https://gem5-review.googlesource.com/12073
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>