systemc: Track exports and prim channels, and call their callbacks.

systemc: Track exports and prim channels, and call their callbacks.

Also call the callbacks on the ports which were already being tracked.

Change-Id: I5ba8ea366e87fc48b58712f35b93c27bccf92cb3
Reviewed-on: https://gem5-review.googlesource.com/12210
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>