systemc: Switch to using predefined messages for core.

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

Change-Id: I05b1398933f753946d5917f39d0f39c7cb45ed9f
Reviewed-on: https://gem5-review.googlesource.com/c/13323
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Remove references to internal constants from the tests.

The tests were referring to constants defined in the Accellera systemc
implementation which identified various log message types. This change
replaces those (sometimes quite long) string constants with their
actual value. This doesn't make that interface any more fragile since
the constant value won't track between the Accellera version and this
one, but it does make its fragility more explicit by using the value
directly.

If in the future we decide to hide the magical nature of those
particular string constants, we could make them into our own constants
in the implementation. Regardless, the tests shouldn't refer to
internal details of the implementation.

Change-Id: I116a7407fcac49367649b250683117f3ccf4f9ed
Reviewed-on: https://gem5-review.googlesource.com/11181
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Import tests from the Accellera systemc distribution.

Change-Id: Iad76b398949a55d768a34d027a2d8e3739953da6
Reviewed-on: https://gem5-review.googlesource.com/10845
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>