py: Add support for _ in REPL to hold last computed value.

Only available when MICROPY_CAN_OVERRIDE_BUILTINS is enabled.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index dcde1b5..161d231 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -35,6 +35,7 @@
 
 Q()
 Q(*)
+Q(_)
 Q(__build_class__)
 Q(__class__)
 Q(__doc__)