all: simplify build tags involving iOS

Prior to Go 1.16, iOS used GOOS=darwin,
so we had to distinguish macOS from iOS during GOARCH.

We now require Go 1.16 in our go.mod, so we can simplify.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

portlist/netstat_exec.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (windows || freebsd || openbsd || (darwin && go1.16) || (darwin && !go1.16 && !arm64)) && !ios
-// +build windows freebsd openbsd darwin,go1.16 darwin,!go1.16,!arm64
+//go:build (windows || freebsd || openbsd || darwin) && !ios
+// +build windows freebsd openbsd darwin
 // +build !ios
 
 package portlist