portlist, version: update build tags for Go 1.16, Apple M1

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>
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