scons: When importing SimObjects, use a copy of globals().

Use a copy of globals when importing SimObjects, instead of the real
deal that the SConscript runs under. This avoids polluting the
SConscript namespace.

Change-Id: I689d693d93cc8998be0967ec2b0c58f5851d39cf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49407
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -594,12 +594,13 @@ m5.params.clear()
 importer = M5MetaPathFinder(PySource.modules)
 sys.meta_path[0:0] = [ importer ]
 
+import_globals = globals().copy()
 # import all sim objects so we can populate the all_objects list
 # make sure that we're working with a list, then let's sort it
 gem5_lib_simobjects = SimObject.all.with_tag(env, 'gem5 lib')
 gem5_lib_modnames = sorted(map(lambda so: so.modname, gem5_lib_simobjects))
 for modname in gem5_lib_modnames:
-    exec('from m5.objects import %s' % modname)
+    exec('from m5.objects import %s' % modname, import_globals)
 
 # we need to unload all of the currently imported modules so that they
 # will be re-imported the next time the sconscript is run