In particular, make Alignof work more like Sizeof. Other idiomatic
cleanups while here.

Passes toolstash -cmp.

Change-Id: I4def20894f3d95e49ab6a50ddba189be36fdd258
Reviewed-on: https://go-review.googlesource.com/20555
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>