systemc: Partially implement SC_FORK and SC_JOIN.

These macros need to expand to some minimal amount of wrapping code to
make the regression tests syntactically legal and compile.

Change-Id: I0b5569704b129d9c315526fc3363ef846a1b5c65
Reviewed-on: https://gem5-review.googlesource.com/11184
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>