cmd/dist: restore GOARM=7 default for android/arm

Fixes the android/arm builder. Without it, the builder reported
unexpected stale targets during bootstrap:

https://build.golang.org/log/b951f1171be54cf4a12c2a0720ffaf07f8a11377

Tighten the GOARM=7 default in cmd/internal/objabi while here.

Change-Id: I944744910193e72e91bc37b5bf0783076b45e579
Reviewed-on: https://go-review.googlesource.com/c/go/+/273167


Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Elias Naur <mail@eliasnaur.com>