This gets us around the kernel helpers on ARMv7.

It is slightly faster than using the kernel helper.

name           old time/op  new time/op  delta
AtomicLoad-4   72.5ns ± 0%  69.5ns ± 0%  -4.08%  (p=0.000 n=9+9)
AtomicStore-4  57.6ns ± 1%  54.4ns ± 0%  -5.58%  (p=0.000 n=10+9)
[Geo mean]     64.6ns       61.5ns       -4.83%

If performance is really critical, we can even do compiler intrinsics
on GOARM=7.

Fixes #23792.

Change-Id: I36497d880890b26bdf01e048b542bd5fd7b17d23
Reviewed-on: https://go-review.googlesource.com/94076
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>