Those new instructions have acquire/release semantics, besides
normal atomic SWPD/SWPW/SWPH/SWPB.

Change-Id: I24821a4d21aebc342897ae52903aef612c8d8a4a
Reviewed-on: https://go-review.googlesource.com/128476
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>