cmd/dist: add windows/arm64 support

- Add Windows SystemInfo constant for arm64
- Add windows/arm64 to GOOS/GOARCH list

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: I6109bd87512b5cb1d227d7a85fd0ac20eb2259e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/288819


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>