[release-branch.go1.14] cmd/cgo: use consistent tag for a particular struct

For #31891
For #38408
Fixes #38426

Change-Id: Ie7498c2cab728ae798e66e7168425e16b063520e
Reviewed-on: https://go-review.googlesource.com/c/go/+/228102


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit 33ff63da)
Reviewed-on: https://go-review.googlesource.com/c/go/+/228107
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>