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>

systemc: Add the Accellera implementation for the data type classes.

These files have been cleaned up style wise, and some macros have been
resolved like they were for the header files.

Change-Id: I447e5311961036847e7da0c5a86c0da25a633010
Reviewed-on: https://gem5-review.googlesource.com/10844
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>