systemc: Partially implement the sc_signal class template.

systemc: Partially implement the sc_signal class template.

There are a few details of this class that still need to be fleshed
out, but this implements most of it.

Change-Id: I6f8b546aacd8537d4341dc91e59b95864ae0f7c4
Reviewed-on: https://gem5-review.googlesource.com/12075
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>