os: Root.Mkdir creates directories with zero permissions on OpenBSD [1.24 backport]

[gopherbot]

@jrick requested issue #73559 to be considered for backport to the next 1.24 minor release.

@gopherbot please consider this for backport to 1.24, it prevents Root.Mkdir from working at all considering that Root also has no Chmod method.

[neild]

I support this backport: It is low-risk and there is no workaround for the bug.

[gopherbot]

Change https://go.dev/cl/669397 mentions this issue: [release-branch.go1.24] os: fix Root.Mkdir permission bits on OpenBSD

[gopherbot]

Closed by merging CL 669397 (commit 431f75a) to release-branch.go1.24.