cmd/compile/internal/gc: assert size of important types

cmd/compile/internal/gc: assert size of important types

Add tests to ensure that the size of important types don't change
unexpectedly.

Skip the test on nacl platforms because of their unusual padding
requirements.

Change-Id: Iddb127a99499e089a309b721f5073356c0da8b24
Reviewed-on: https://go-review.googlesource.com/20285
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>