systemc: Add some nonstandard SC_WAIT* macros.

These are not in the standard, but are defined by Accellera and used in
the tests.

Change-Id: I4cbddac4eb75c58d8ae3eb77d1f291dac3c51cd6
Reviewed-on: https://gem5-review.googlesource.com/11261
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>