runtime: fix vet complaints for linux/amd64

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

This CL makes "GOOS=linux GOARCH=amd64 go vet -unsafeptr=false runtime" happy,
while keeping "GO_BUILDER_NAME=misc-vetall go tool dist test" happy too.

For #31916.

Change-Id: I4ca1acb02f4666b102d25fcc55fac96b8f80379a
Reviewed-on: https://go-review.googlesource.com/c/go/+/176100


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>