internal/syscall/unix: implement Eaccess on openbsd

internal/syscall/unix: implement Eaccess on openbsd

Like on other BSDs, use faccessat(AT_FDCWD, path, mode, AT_EACCESS)

Change-Id: I80f8d327dd152576165b9206e32dfb749b41d187
Reviewed-on: https://go-review.googlesource.com/c/go/+/538836
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>