Build tags have been updated to build native Apple M1 binaries, existing build
tags for ios have been changed from darwin,arm64 to ios,arm64.
With this change, running go build cmd/tailscale{,d}/tailscale{,d}.go on an Apple
machine with the new processor works and resulting binaries show the expected
architecture, e.g. tailscale: Mach-O 64-bit executable arm64.
Tested using go version go1.16beta1 darwin/arm64.
Updates #943
Signed-off-by: moncho <50428+moncho@users.noreply.github.com>
