cmd/dist: detect GOHOSTARCH on iOS

cmd/dist defaults to GOHOSTARCH=amd64 on darwin because no other
darwin host could build Go. With the upcoming self-hosted iOS
builders, GOHOSTARCH=arm64 is also possible.

Updates #31722

Change-Id: I9af47d9f8c57ea45475ce498acefbfe6bf4815b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174306


Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>