The export data format was augmented with a new "unsafe-uintptr" tag
in https://golang.org/cl/18584, but builtin.go was not regenerated.

While here, add a test to make sure builtin.go stays up to date in the
future.

Change-Id: I4ae17da29f0855bef6ec0fcc10e7082c8427d39c
Reviewed-on: https://go-review.googlesource.com/19681
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>