systemc: Fill out sc_process_handle and create Process classes.

The sc_process_handle class now primarily delegates to a Process object
it points at. The Process object does book keeping as far as its
internal state, but doesn't yet have a way to run its target function
or to schedule itself or inject exceptions into its context of
execution.

Change-Id: I98389778abe29aa26e3e3a91bf02e6721acc8a9c
Reviewed-on: https://gem5-review.googlesource.com/11613
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/core/SConscript

@@ -33,6 +33,7 @@ if env['USE_SYSTEMC']:
     Source('kernel.cc')
     Source('module.cc')
     Source('object.cc')
+    Source('process.cc')
 
     Source('sc_attr.cc')
     Source('sc_event.cc')