systemc: Distinguish internal events from normal sc_events.

The internal events aren't supposed to show up in the namespace or as
children of objects.

Change-Id: Id04b9bfe2e1f8f216390dd989797558eaf33d715
Reviewed-on: https://gem5-review.googlesource.com/c/13309
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement SC_FORK, SC_JOIN, and SC_CJOIN.

SC_CJOIN is non-standard, but relied on by the Accellera tests.

Change-Id: Ia4ddcb1749a07891157a58398137e94fcaa8e815
Reviewed-on: https://gem5-review.googlesource.com/c/12615
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add the nonstandard sc_join class and sc_thread_handle type.

Change-Id: I09905bad4797d9c456229afe601006ce16977394
Reviewed-on: https://gem5-review.googlesource.com/11353
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>