systemc: Add the nonstandard, undocumented halt function.

Used in the tests.

Change-Id: I9b4f10600a50e0def1b5d55428cb4ad49e401295
Reviewed-on: https://gem5-review.googlesource.com/11263
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>