Mark atomic LL/SC loops as unsafe for async preemption, as they
use REGTMP.

Change-Id: I5be7f93ad3ee337049ec7c3efd6fdc30eef87d97
Reviewed-on: https://go-review.googlesource.com/c/go/+/203719
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>