scons: Add sim_object and enums arguments to SimObject().

This will explicitly declare what SimObject and Enum types need to be set
up in C++, which will make importing all the SimObject modules during
the setup phase of SCons uneccessary.

Change-Id: Id2d7603daf33b236ceaa0789e2f089f589d34e62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49406
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/systemc/core/SConscript

@@ -28,7 +28,8 @@ import gem5_scons
 Import('*')
 
 if env['USE_SYSTEMC']:
-    SimObject('SystemC.py')
+    SimObject('SystemC.py', sim_objects=[
+        'SystemC_Kernel', 'SystemC_ScObject', 'SystemC_ScModule'])
 
     Source('channel.cc')
     Source('event.cc')