Followup to previous typenod CL. Changes export data format, but only
the compiler-specific section, so no version bump.

Change-Id: I0c21737141f3d257366b29b2a9211bc7217c39ee
Reviewed-on: https://go-review.googlesource.com/39797
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>