gc: make string const comparison unsigned

Make compile-time string const comparison match semantics
of runtime.cmpstring.

Fixes #1515.

R=rsc
CC=golang-dev, rog
https://golang.org/cl/4172049

src/cmd/gc/const.c

@@ -1051,12 +1051,12 @@ int
 cmpslit(Node *l, Node *r)
 {
 	int32 l1, l2, i, m;
-	char *s1, *s2;
+	uchar *s1, *s2;
 
 	l1 = l->val.u.sval->len;
 	l2 = r->val.u.sval->len;
-	s1 = l->val.u.sval->s;
-	s2 = r->val.u.sval->s;
+	s1 = (uchar*)l->val.u.sval->s;
+	s2 = (uchar*)r->val.u.sval->s;
 
 	m = l1;
 	if(l2 < m)

test/fixedbugs/bug1515.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+package main
+
+const (
+	joao = "João"
+	jose = "José"
+)
+
+func main() {
+	s1 := joao
+	s2 := jose
+	if (s1 < s2) != (joao < jose) {
+		panic("unequal")
+	}
+}