systemc: Refactor reporting to prep for int based messages.

systemc: Refactor reporting to prep for int based messages.

There's a deprecated reporting mechanism based on integer message ids,
and the reporting mechanism needs to be refactored a bit to make it
easier to support.

Some bookkeeping data structures were moved out to somewhere they
can be accessed by other code, obviating the non-standard get_handler
function.

Change-Id: Id427cd79be9ef0f3275fbac39ff047ab672fb3e0
Reviewed-on: https://gem5-review.googlesource.com/c/13318
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>