all: prepare for GOOS=ios in Go 1.16

Work with either way for now on iOS (darwin/arm64 vs ios/arm64).

In February when Go 1.16 comes out we'll have a universal binary for
darwin/arm64 (macOS) and will drop support for Go 1.15 and its
darwin/amd64 meaning iOS. (it'll mean macOS).

Context:

* https://tip.golang.org/doc/go1.16#darwin
* https://github.com/golang/go/issues/38485
* https://github.com/golang/go/issues/42100

version/prop.go

@@ -10,17 +10,16 @@ import "runtime"
 func IsMobile() bool {
 	// Good enough heuristic for now, at least until Apple makes
 	// ARM laptops...
-	return runtime.GOOS == "android" ||
-		(runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64"))
+	return runtime.GOOS == "android" || isIOS
 }
 
 // OS returns runtime.GOOS, except instead of returning "darwin" it
 // returns "iOS" or "macOS".
 func OS() string {
+	if isIOS {
+		return "iOS"
+	}
 	if runtime.GOOS == "darwin" {
-		if IsMobile() {
-			return "iOS"
-		}
 		return "macOS"
 	}
 	return runtime.GOOS

version/prop_ios.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ios
+
+package version
+
+const isIOS = true

version/prop_notios.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !ios
+
+package version
+
+const isIOS = false