os: fix Root.Mkdir permission bits on OpenBSD
Pass missing mode bits in the mkdirat() syscall wrapper.
Fixes #73559
Change-Id: I54b1985bd77b1fe5d1a48acab9f2597f8c931854
GitHub-Last-Rev:
669c17361d86bc9065bb6b47a2d60aa86bcfa12d
GitHub-Pull-Request: golang/go#73565
Reviewed-on: https://go-review.googlesource.com/c/go/+/669375
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>