runtime: fix vet complaints for linux/arm64, linux/mips*, linux/ppc64*, linux/s390x

runtime: fix vet complaints for linux/arm64, linux/mips*, linux/ppc64*, linux/s390x

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 GOOS/GOARCHes,
except for an unresolved complaint on mips/mipsle that is a bug in vet,
while keeping "GO_BUILDER_NAME=misc-vetall go tool dist test" happy too.

For #31916.

Change-Id: I6ef7e982a2fdbbfbc22cee876ca37ac54d8109e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/176102
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>