os: make UserHomeDir return "/" on iOS

The UserHomeDir test succeeds on the builder, but not when run
manually where HOME is set to the host $HOME.

Change-Id: I1db0f608b04b311b53cc0c8160a3778caaf542f6
Reviewed-on: https://go-review.googlesource.com/c/141798
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -394,6 +394,11 @@ func UserHomeDir() string {
 		return Getenv("home")
 	case "nacl", "android":
 		return "/"
+	case "darwin":
+		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
+			return "/"
+		}
+		fallthrough
 	default:
 		return Getenv("HOME")
 	}