Add a variant of sync/atomic's TestUnaligned64 to
runtime/internal/atomic.

Skips the test on arm for now where it's currently failing.

Updates #17786

Change-Id: If63f9c1243e9db7b243a95205b2d27f7d1dc1e6e
Reviewed-on: https://go-review.googlesource.com/33159
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>