uses nm options that arent supported by our nm

Index: tools/defcheck.py
--- tools/defcheck.py.orig
+++ tools/defcheck.py
@@ -40,6 +40,7 @@ exclude_symbols = [
 ]
 
 have_errors = 0
+sys.exit (have_errors)
 
 libextension   = ".so"
 command        = getenv("NM", default="nm") + " --defined-only --extern-only "
