systemc: Switch to using predefined messages for datatypes.

Create and use predefined messages for datatypes which match the ones
Accellera uses.

Change-Id: I92dd52f62462b864264217bb81f3ff1dcec020bf
Reviewed-on: https://gem5-review.googlesource.com/c/13331
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>