systemc: Switch to using predefined messages for utils.

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

Change-Id: I932b7206fc16181d01a0d5b7441ce617b30e5365
Reviewed-on: https://gem5-review.googlesource.com/c/13328
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>