Updates #20650 (CL 45099 introduced the feature to x86).

Change-Id: If40cc9d87417a05281d8633f05cd91f6f434b136
Reviewed-on: https://go-review.googlesource.com/80843
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>