systemc: Add some "implementation defined" stuff to sc_attr_cltn.

systemc: Add some "implementation defined" stuff to sc_attr_cltn.

These "impelementation defined" methods are tested by the regression
tests, so we need to have them. We might as well have the same
general interface as the Accellera implementation since nothing there
seems overly specialized for that environment.

Change-Id: Ief6567fcd9d99d3a0f526cfa3b65043b2c828efa
Reviewed-on: https://gem5-review.googlesource.com/11185
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>