systemc: Fix the implementation of sc_event::triggered.

systemc: Fix the implementation of sc_event::triggered.

This function just returned false. The new implementation uses the
scheduler's changeStamp function to keep track of how recently the
event was triggered so it can return return the right value.

Change-Id: Idf61cd459e53e224a33d3e53c16210f5e0a99a3a
Reviewed-on: https://gem5-review.googlesource.com/c/12825
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>