systemc: Add a stubbed out implementation of the sc_sensitive class.

systemc: Add a stubbed out implementation of the sc_sensitive class.

This sc_sensitive class is mostly implementation defined, but has a
few standards defined methods.

Change-Id: I7157f6bfaaef38b5804b19a1de9f3f0aff08b697
Reviewed-on: https://gem5-review.googlesource.com/10833
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>