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>
2 files changed