Added a flag to generic and various architectures' atomic
operations that are judged to have observable side effects
and thus cannot be dead-code-eliminated.

Test requires GOMAXPROCS > 1 without preemption in loop.

Fixes #19182.

Change-Id: Id2230031abd2cca0bbb32fd68fc8a58fb912070f
Reviewed-on: https://go-review.googlesource.com/39595
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>