Kick start the upstreaming of the arm64 port. The only manual
change is cmd/go/pkg.go.

Change-Id: I0607ad045486f0998c4e21654b59276ca5348069
Reviewed-on: https://go-review.googlesource.com/7075Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Run-TryBot: Aram Hăvărneanu <aram@mgk.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>