Passes toolstash/buildall.

Fixes #14692.

Change-Id: I4352678d8251309f2b8b7793674c550fac948006
Reviewed-on: https://go-review.googlesource.com/20350Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>