cmd/ld: handle IMAGE_SYM_CLASS_LABEL symbols.

Was causing "invalid symbol binding" and thus "malformed pe file" error.

R=golang-dev, alex.brainman, adg
CC=golang-dev
https://golang.org/cl/13722050

src/cmd/ld/ldpe.c

@@ -468,6 +468,7 @@ readsym(PeObj *obj, int i, PeSym **y)
 			break;
 		case IMAGE_SYM_CLASS_NULL:
 		case IMAGE_SYM_CLASS_STATIC:
+		case IMAGE_SYM_CLASS_LABEL:
 			s = lookup(name, version);
 			s->dupok = 1;
 			break;