cmd/dist: recognize darwin/arm as (host) goos/goarches. also hard
          code GOARM=7 for darwin/arm.
make.bash: don't pass -mmacosx-version-min=10.6 when building for
           darwin/arm.

Change-Id: If0ecd84a5179cd9bb61b801ac1899adc45f12f75
Reviewed-on: https://go-review.googlesource.com/2126Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>