systemc: Add a minimal version of the deprecated sc_process_b.

This type is deprecated, but some tests still rely on it. This change
adds just enough of it to satisfy the tests, and also the several
different mechanisms for retrieving the sc_process_b which refers to
the currently active process.

Change-Id: Id122ae5df23744b5de1e1c97573412f97a73b77a
Reviewed-on: https://gem5-review.googlesource.com/11351
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
4 files changed