Ignore respective bit in export data, but leave the info to
minimize format changes for 1.7. Scheduled to remove by 1.8.

For #15772.

Change-Id: Ifb3beea655367308a4e2d5dc8cb625915f904287
Reviewed-on: https://go-review.googlesource.com/23285
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>