F_OK is used with access(2), but it was removed for openbsd/amd64 when
generated code was updated to OpenBSD 6.3 in 7f59abf3.

Fixes golang/go#26593

Change-Id: I4a542506b1045f0c860bb3137c4d81149a5e1cae
Reviewed-on: https://go-review.googlesource.com/126635
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>