Auto merge of #115108 - ijackson:broken-wait-status, r=dtolnay

Fix exit status / wait status on non-Unix cfg(unix) platforms

Fixes #114593

Needs FCP due to behavioural changes (NB only on non-Unix `#[cfg(unix)]` platforms).

Also, I think this is likely to break in CI.  I have not been yet able to compile the new bits of `process_unsupported.rs`, although I have compiled the new module.  I'd like some help from people familiar with eg emscripten and fuchsia (which are going to be affected, I think).

Author: bors