scons: Make the sim_objects parameter of SimObject mandantory.

If there really are no c++ sim_objects in the file, then sim_objects can
be set to [] which it used to default to.

This way, if someone hasn't remembered to update their SConscript files
for the new sim_objects and enums parameters, this will give them some
indication what's wrong, rather than the build just failing later.

Change-Id: Ic1933f7b9dfff7dd7e403c6c84f1f510c8ee8c72
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54203
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/SConscript

@@ -149,13 +149,21 @@ class SimObject(PySource):
     enums = dict()
     tags = dict()
 
-    def __init__(self, source, *, sim_objects=[], enums=[],
+    def __init__(self, source, *, sim_objects=None, enums=None,
             tags=None, add_tags=None):
         '''Specify the source file and any tags (automatically in
         the m5.objects package)'''
+        if sim_objects is None:
+            if enums is None:
+                error(f"SimObject({source}...) must list c++ sim_objects or "
+                      "enums (set either to [] if there are none).")
+            sim_objects = []
+        if enums is None:
+            enums = []
+
         super().__init__('m5.objects', source, tags, add_tags)
         if self.fixed:
-            raise AttributeError("Too late to call SimObject now.")
+            error("Too late to call SimObject now.")
 
         SimObject.sim_objects[self.modpath] = sim_objects
         SimObject.enums[self.modpath] = enums