systemc: Differentiate between notifying methods and threads.

systemc: Differentiate between notifying methods and threads.

The Accellera implementation notifies all types of method
sensitivities first, and then notifies all the ones for threads.

Change-Id: I5eda75958675ba518f008852148030e032f70d83
Reviewed-on: https://gem5-review.googlesource.com/c/12807
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>