all: gofmt with Go 1.17

This adds "//go:build" lines and tidies up existing "// +build" lines.

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

version/cmdname_ios.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build (go1.16 && ios) || (!go1.16 && darwin && arm64)
 // +build go1.16,ios !go1.16,darwin,arm64
 
 package version