systemc: Add the deprecated sc_signal_out_if typedef.

This is needed by the regression tests.

Change-Id: I5666cf9ad445869e74edda857afd59ab7ece4f4c
Reviewed-on: https://gem5-review.googlesource.com/10955
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Stub out the predefined channels.

Change-Id: Ie030aad26875bd49e54981ec1e9076b7b5af6630
Reviewed-on: https://gem5-review.googlesource.com/10839
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>