Allows removing fields that aren't relevant to a particular OS or
changing their types to match the underlying OS system calls they'll
be used for.

Change-Id: I5cea89ee77b4e7b985bff41337e561887c3272ff
Reviewed-on: https://go-review.googlesource.com/16176Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>