Implement getexepath for OpenBSD

[am11]

Contributes to #124911.

[am11]

cc @janvorli, @sethjackson, minipal_getexepath is a best effort API on OpenBSD, as the platform intentionally lacks a direct kernel interface (like FreeBSD's KERN_PROC_PATHNAME) for the absolute executable path.

argv is normally fine to get the executable path (https://stackoverflow.com/q/31494901). I have tested it on the VM with scenarios like 'running from same dir' and 'running from different dir' and realpath does the job well in the end.

The only downside of relying on argv is that if someone spawns this process and manually rewrites argv to a different/dummy value, then it will get that val (but it's not a sane thing to do so we shouldn't worry 😅).

[janvorli]

LGTM, thank you!