systemc: Turn some macros into enums to fix regression test building.

systemc: Turn some macros into enums to fix regression test building.

The spec defines these constants to be macros, but the regression test
refers to them using the sc_core namespace. That breaks things because
the macros don't expand to an identifier, they expand to an expression.

This change converts the macros into enums like in the Accellera
implementation to get the regression tests to build.

Change-Id: I3db7dc87a7a597907df658624a7343bc6ca1734e
Reviewed-on: https://gem5-review.googlesource.com/11180
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>