5l, 6l, 8l: implement -X flag

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5643050
diff --git a/test/linkx.go b/test/linkx.go
new file mode 100644
index 0000000..caa815a3
--- /dev/null
+++ b/test/linkx.go
@@ -0,0 +1,15 @@
+// $G $D/$F.go && $L -X main.tbd hello $F.$A && ./$A.out
+
+// Copyright 2012 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
+
+var tbd string
+
+func main() {
+	if tbd != "hello" {
+		println("BUG: test/linkx", len(tbd), tbd)
+	}
+}