runtime: fix vet complaints for all freebsd, netbsd, openbsd

Working toward making the tree vet-safe instead of having
so many exceptions in cmd/vet/all/whitelist.

This CL makes "go vet -unsafeptr=false runtime" happy for these GOOSes,
while keeping "GO_BUILDER_NAME=misc-vetall go tool dist test" happy too.

For #31916.

Change-Id: I63c4805bdd44b301072da66c77086940e2a2765e
Reviewed-on: https://go-review.googlesource.com/c/go/+/176105


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>