On amd64p32, PtrSize and RegSize don't agree, and function return
value is aligned with RegSize. Fix this rule. Other architectures
are not affected, where PtrSize and RegSize are the same.

Change-Id: If187d3dfde3dc3b931b8e97db5eeff49a781551b
Reviewed-on: https://go-review.googlesource.com/37720
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>