systemc: Add the nonstandard macro SC_NEW.

This is in the Accellera implementation and in the regression tests.
The implementation here is a bit different than theirs in that it uses
std::unique_ptrs.

Change-Id: Id3d1ad82482b94a5d99f27e02d1e447ca1944797
Reviewed-on: https://gem5-review.googlesource.com/11255
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>