Support OpenBSD

[kpcyrd]

I'd be very interested in an OpenBSD target :)

tokio-rs/mio#593

[japaric]

This is at least blocked on having official releases of the rust-std component for OpenBSD. In other words, your target should show up in the output of rustup target list. If it doesn't, you should request binary releases of std in the rust-lang/rust issue tracker.

[Alexhuszagh]

Since all the OpenBSD targets are tier 3, and they seem to have only stayed at GCC 4.2.1 (pre-C++11, I have no idea if they supported later), and binutils ld does not support the OpenBSD targets, this may be a bit of work. Clang is the obvious solution, but as it's a tier 3 target, I'm less interesting in adding support for it as opposed to other ones. If you would like to help, that would be more than welcome. The latest documentation I can see on supporting it is from 8 years ago. This is the current state of my work on it, but it's extremely rudimentary and still assumes binutils/GCC will work on it (which for any recent version, they won't).