systemc: Switch to using predefined messages for channels.

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

Change-Id: I179214838bbd83604e50225926cdc6b5b1b16923
Reviewed-on: https://gem5-review.googlesource.com/c/13330
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement writer policies.

This includes the nonstandard SC_NO_WRITE_CHECK #define which the
Accellera tests use and depend on.

Change-Id: I106c4c16160325725a00d5c337047251817dca32
Reviewed-on: https://gem5-review.googlesource.com/c/13203
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement register_port in all the predefined channels.

Something the Accellera implementation does which would be good to do
in the gem5 implementation is to create a base class for sc_signal
which isn't templated, and which holds the common/non-type specific
versions of the various sc_signal methods. This will reduce code
redundancy and binary size, and also let us hide more code in .cc
files so that it's less likely we'd need to recompile model code to
fix a bug.

Also, since this all uses of sc_channel_warn_unimple have now been
eliminated, remove that function.

Change-Id: Ia574647c034e7136093c2047b69de725ac34f52f
Reviewed-on: https://gem5-review.googlesource.com/c/13200
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Stub out the predefined channels.

Change-Id: Ie030aad26875bd49e54981ec1e9076b7b5af6630
Reviewed-on: https://gem5-review.googlesource.com/10839
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>