cmd/go: let go tool invocation use stdin

Fixes #2921.

R=golang-dev, n13m3y3r
CC=golang-dev
https://golang.org/cl/5647057

src/cmd/go/tool.go

@@ -70,6 +70,7 @@ func runTool(cmd *Command, args []string) {
 	toolCmd := &exec.Cmd{
 		Path:   toolPath,
 		Args:   args,
+		Stdin:  os.Stdin,
 		Stdout: os.Stdout,
 		Stderr: os.Stderr,
 	}