[gamecode] Add format for addressing modes and use in return

This fixes Ruamoko's return format string. It looks like it's producing
the correct address (but doesn't show all the information it should),
but the rest of the debug code needs work locals.

libs/gamecode/opcodes.py

@@ -463,7 +463,7 @@ return_formats = {
     "mnemonic": "return",
     "opname": "return",
     "widths": "-1, -1, 0",    # width specified by st->c
-    "format": "FIXME",
+    "format": "%Mc5",
     "types": "ev_void, ev_void, ev_void",
 }
 vecops_formats = {