gc: escape unicode BOM in exported string literals

Fixes #5260.

R=golang-dev, minux.ma, 0xjnml, r
CC=golang-dev
https://golang.org/cl/8658043

src/cmd/gc/fmt.c

@@ -443,6 +443,9 @@ Zconv(Fmt *fp)
 			fmtrune(fp, '\\');
 			fmtrune(fp, r);
 			break;
+		case 0xFEFF: // BOM, basically disallowed in source code
+			fmtstrcpy(fp, "\\uFEFF");
+			break;
 		}
 	}
 	return 0;

test/fixedbugs/issue5260.dir/a.go

+// Copyright 2013 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 a
+
+const BOM = "\uFEFF"

test/fixedbugs/issue5260.dir/b.go

+// Copyright 2013 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
+
+import "./a"
+
+func main() {
+	_ = a.BOM
+}

test/fixedbugs/issue5260.go

+// rundir
+
+// Copyright 2013 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 5260: Unicode BOM in exported string constant
+// cannot be read back during package import.
+
+package ignored