systemc: Add a nonstandard sc_signal constructor.

This constructor takes an initial value to set the sc_signal to, and
is used in the tests.

Change-Id: I197218846d9a79f9237238c78b1bbd8a7f55443f
Reviewed-on: https://gem5-review.googlesource.com/11266
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>