systemc: Distinguish internal events from normal sc_events.

The internal events aren't supposed to show up in the namespace or as
children of objects.

Change-Id: Id04b9bfe2e1f8f216390dd989797558eaf33d715
Reviewed-on: https://gem5-review.googlesource.com/c/13309
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement sc_semaphore.

Change-Id: I778d41bd81880e76caa71dc92359a00127d8f987
Reviewed-on: https://gem5-review.googlesource.com/c/12617
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>