cmd/gc: ignore blank (_) labels in label declarations

Fixes #7538

LGTM=rsc
R=gri, rsc
CC=golang-codereviews
https://golang.org/cl/85040045

src/cmd/gc/gen.c

@@ -301,6 +301,10 @@ gen(Node *n)
 		break;
 
 	case OLABEL:
+		if(isblanksym(n->left->sym)) {
+			break;
+		}
+		
 		lab = newlab(n);
 
 		// if there are pending gotos, resolve them all to the current pc.

test/fixedbugs/issue7538a.go

+// errorcheck
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 7538: blank (_) labels handled incorrectly
+
+package p
+
+func f() {
+_:
+_:
+	goto _ // ERROR "not defined"
+}

test/fixedbugs/issue7538b.go

+// compile
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 7538: blank (_) labels handled incorrectly
+
+package p
+
+func f() {
+_:
+}