cmd/dist: handle arm64 as a machine hardware name

OpenBSD/arm64 reports itself as arm64 from `uname -m` - this currently
matches as gohostarch=arm, rather than gohostarch=arm64. Correct this
by matching on both aarch64 and arm64 (the alternative would be to use
`uname -p`, however that's likely to cause upset elsewhere).

Updates #31656

Change-Id: I7b2da93495d808e704caf032b3fadc984077769e
Reviewed-on: https://go-review.googlesource.com/c/go/+/173598


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>