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/SConscript

@@ -142,7 +142,8 @@ class SimObject(PySource):
 
     fixed = False
 
-    def __init__(self, source, tags=None, add_tags=None):
+    def __init__(self, source, *, sim_objects=[], enums=[],
+            tags=None, add_tags=None):
         '''Specify the source file and any tags (automatically in
         the m5.objects package)'''
         super().__init__('m5.objects', source, tags, add_tags)