systemc: Add some nonstandard (get|set)_catch_actions functions.

These are used by the tests.

Change-Id: I09bbe81854f0faa524a4c6aa0ef31d80e4a8de24
Reviewed-on: https://gem5-review.googlesource.com/11265
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>