Fix help message printing. Might need to clean up the handling of the sys.exit() call, as right now it prints out "None" at the end (not sure why).

src/python/m5/main.py:
    Fix help message printing.

--HG--
extra : convert_revision : 6906234101eb7ff7df7933e9aede0362b5a991bd

src/python/m5/main.py

@@ -182,7 +182,7 @@ options = attrdict()
 arguments = []
 
 def usage(exitcode=None):
-    print parser.help
+    print parser.print_help()
     if exitcode is not None:
         sys.exit(exitcode)