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: Remove some old warn includes from ext/channel.

The calls to sc_channel_warn_unimpl have been removed, so the includes
aren't needed.

Change-Id: I58a916a7c4ef9a266be591789696457799b6b8dd
Reviewed-on: https://gem5-review.googlesource.com/c/13198
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Change an error message to match the reference outputs.

This is different from the message the Accellera implementation would
print, so it looks like it wouldn't pass this test as is.

Change-Id: I0286fd3e3df7bc3e87f38c35086a32d8dec0bcc9
Reviewed-on: https://gem5-review.googlesource.com/c/12814
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement the sc_signal_rv channel and ports.

Change-Id: Id1a3fd2ded224bbe94a4a65e0acf34a3547aedcc
Reviewed-on: https://gem5-review.googlesource.com/c/12813
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>