cmd/dist: fix wrong goarch on mips64le

Change-Id: I625f0bc533a7d14010c0344f36e8f157a19c13f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/203437


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>