Directly accept GOARM env var for ARM version

As GOARCH defaults to 'arm' on arm systems this allows packagers to
specify the arm version by setting the GOARM env var to 5, 6 or 7.

build.go

@@ -75,8 +75,14 @@ func main() {
 	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.")
+		switch os.Getenv("GOARM") {
+		case "5", "6", "7":
+			goarch += "v" + os.Getenv("GOARM")
+			break
+		default:
+			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)
 	}