runtime: add TestSizeof

Borrowed from cmd/compile, TestSizeof ensures
that the size of important types doesn't change unexpectedly.
It also helps reviewers see the impact of intended changes.

Change-Id: If57955f0c3e66054de3f40c6bba585b88694c7be
Reviewed-on: https://go-review.googlesource.com/99837


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>