gc: implement and test \r in raw strings

For issue 680.

R=ken2
CC=golang-dev
https://golang.org/cl/5492046

src/cmd/gc/lex.c

@@ -840,6 +840,8 @@ l0:
 				ncp += ncp;
 			}
 			c = getr();
+			if(c == '\r')
+				continue;
 			if(c == EOF) {
 				yyerror("eof in string");
 				break;

test/crlf.go

+// $G $D/$F.go && $L $F.$A && ./$A.out >tmp.go &&
+// $G tmp.go && $L tmp.$A && ./$A.out
+// rm -f tmp.go
+
+// 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.
+
+// Test source files and strings containing \r and \r\n.
+
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+func main() {
+	prog = strings.Replace(prog, "BQ", "`", -1)
+	prog = strings.Replace(prog, "CR", "\r", -1)
+	fmt.Print(prog)
+}
+
+var prog = `
+package main
+CR
+
+import "fmt"
+
+var CR s = "hello\n" + CR
+	" world"CR
+
+var t = BQhelloCR
+ worldBQ
+
+var u = BQhCReCRlCRlCRoCR
+ worldBQ
+
+var golden = "hello\n world"
+
+func main() {
+	if s != golden {
+		fmt.Printf("s=%q, want %q", s, golden)
+	}
+	if t != golden {
+		fmt.Printf("t=%q, want %q", t, golden)
+	}
+	if u != golden {
+		fmt.Printf("u=%q, want %q", u, golden)
+	}
+}
+`