arm: move rsc.io/arm here

This code was reviewed on its way into rsc.io/arm:

    https://codereview.appspot.com/104770047
    https://codereview.appspot.com/106720044
    https://codereview.appspot.com/101820043
    https://codereview.appspot.com/101800044
    https://codereview.appspot.com/101740045
    https://codereview.appspot.com/93600044

Change-Id: I9076e024aa9a433bedfa2760315dc056bf5c66fc
Reviewed-on: https://go-review.googlesource.com/13971
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
20 files changed