systemc: Add stubbed out versions of sc_event and related classes.

Change-Id: Id45c80cbb8774d8469d4df6ce7915161df977de0
Reviewed-on: https://gem5-review.googlesource.com/10828
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
3 files changed