go/build: add go1.2 build tag

Fixes #6449.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13829044

src/pkg/go/build/build.go

@@ -295,7 +295,7 @@ func defaultContext() Context {
 	// When we reach Go 1.3 the line will read
 	//	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3"}
 	// and so on.
-	c.ReleaseTags = []string{"go1.1"}
+	c.ReleaseTags = []string{"go1.1", "go1.2"}
 
 	switch os.Getenv("CGO_ENABLED") {
 	case "1":