cmd/dist: mark cmd/link and debug/goobj as unreleased

These are not ready and will not be in Go 1.3.

Fixes #6932.

LGTM=bradfitz
R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews, iant, r
https://golang.org/cl/87630043

src/cmd/dist/build.c

@@ -373,6 +373,8 @@ static char *oldtool[] = {
 // Unreleased directories (relative to $GOROOT) that should
 // not be in release branches.
 static char *unreleased[] = {
+	"src/cmd/link",
+	"src/pkg/debug/goobj",
 	"src/pkg/old",
 };