systemc: Add nonstandard sc_trace-s for sc_event and sc_time.

These are not in the standard but are defined by Accellera and are
used in the regression tests.

Change-Id: Idb8358530b1135526c52733e628461a17308e216
Reviewed-on: https://gem5-review.googlesource.com/11259
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>