systemc: Add the nonstandard variable sc_allow_process_control_corners.

systemc: Add the nonstandard variable sc_allow_process_control_corners.

This variable controls what happens in some situations which are left
as undefined in the spec. It's behavior is explained in a big comment
in the Accellera implementation. Since it's used in the regression
tests, we need to at least have that variable so they'll compile and
link properly.

Change-Id: I1ac4592641be3d9dd10e7bf6144704a6fac1b2d4
Reviewed-on: https://gem5-review.googlesource.com/11186
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>