Also, replace "PtrSize == 4 && Arch != amd64p32" with "RegSize == 4".

Passes toolstash-check -all.

Updates #19954.

Change-Id: I79b2ee9324f4fa53e34c9271d837ea288b5d7829
Reviewed-on: https://go-review.googlesource.com/41491
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>