Add OpenBSD support

[lcheylus]

I would like to add support for OpenBSD OS (target_os = "openbsd" for rustc).

I think some features will require some addionnal code in rust-lang/libc.

[GuillaumeGomez]

Indeed. I needed to make quite a lot of changes to libc. But the biggest requirement will be for CI: sysinfo needs a CI running openbsd (preferrably not in a VM).

[lcheylus]

There is no GitHub runner for OpenBSD.

The only solution I know is to use the GH action vmactions/openbsd-vm to use it in CI:

• run OpenBSD VM with this action
• install rustc via OpenBSD package (no rustup support for this OS)
• run build/tests via shell script.

[GuillaumeGomez]

I know. I use CirrusCI for mac M1 and freebsd. So other CIs are also an option.