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 sc_buffer.

This required a small change to sc_signal so that the value change
event and the change stamp for it were accessible.

Change-Id: Ife0545d84f3b25e98da079786c30ffa51025cce7
Reviewed-on: https://gem5-review.googlesource.com/c/12804
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add a non-standard default writer policy on sc_buffer.

The standard says there's not supposed to be a default writer policy
for the sc_buffer template class, but in the Accellera implementation
there is, and the regression tests depend on it to compile.

Change-Id: I31d17617441224e86c56c54e45364be8f4f45b00
Reviewed-on: https://gem5-review.googlesource.com/10957
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>