runtime: fix vet complaints for all nacl

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 nacl/*,
while keeping "GO_BUILDER_NAME=misc-vetall go tool dist test" happy too.

For #31916.

Change-Id: I6adb4a7b0c2b03d901fba37f9c05e74e5b7b6691
Reviewed-on: https://go-review.googlesource.com/c/go/+/176107


Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>