runtime.armArch is unused on linux/arm since CL 93637. The new code in
runtime/internal/atomic (added by CL 111315) only checks runtime.goarm.

Change-Id: Ic097ee6750e39abb20f45770a1c7c2d925f02408
Reviewed-on: https://go-review.googlesource.com/c/140077
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>