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: Stub out all the standard utilility classes and functions.

Change-Id: I9e9724edb6281e0b0a6bae5546b0ede77d295c12
Reviewed-on: https://gem5-review.googlesource.com/10841
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>