Warn about incorrect -goarch values

build.go

@@ -55,6 +55,16 @@ func main() {
 	flag.BoolVar(&noupgrade, "no-upgrade", false, "Disable upgrade functionality")
 	flag.Parse()
 
+	switch goarch {
+	case "386", "amd64", "armv5", "armv6", "armv7":
+		break
+	case "arm":
+		log.Println("Invalid goarch \"arm\". Use one of \"armv5\", \"armv6\", \"armv7\".")
+		log.Fatalln("Note that producing a correct \"armv5\" binary requires a rebuilt stdlib.")
+	default:
+		log.Printf("Unknown goarch %q; proceed with caution!", goarch)
+	}
+
 	checkRequiredGoVersion()
 
 	if check() != nil {