config.py: use correct archs parameter

config.py

@@ -3,7 +3,8 @@ https://cirosantilli.com/linux-kernel-module-cheat#default-command-line-argument
 '''
 
 def set_args(args, script_name):
-    args['arch'] = 'aarch64'
+    # archs in plural here because --arch adds items to a list of archs.
+    args['archs'] = ['aarch64']
     args['emulators'] = ['gem5']
     if script_name == 'build-gem5':
         # This argument is defined only for ./build-gem5.