x/sys/unix: update openbsd to OpenBSD 7.4

[mauri870]

OpenBSD 7.4 is now released

I'm not sure if there is any issues with this release, but as far as I know all the necessary files can be autogerated with mkall.sh in a openbsd 7.4 system.

e.g GOOS=openbsd ./mkall.sh

[cagedmantis]

cc @golang/runtime

[gopherbot]

Change https://go.dev/cl/537355 mentions this issue: unix: require minimum OpenBSD 6.4 for pledge, unveil

[4a6f656c]

There should be limited changes since OpenBSD uses libc based system calls.

[gopherbot]

Change https://go.dev/cl/538995 mentions this issue: unix: use fcntl(2) libc stub on OpenBSD