syscall: fix vet complaints for all dragonfly, freebsd, netbsd, openbsd

syscall: fix vet complaints for all dragonfly, 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: Id2e1223497bd0cd6e880cd81f3ece6363e58219f
Reviewed-on: https://go-review.googlesource.com/c/go/+/176104
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>