Cross-host builds are broken (and nightlies)

[alexcrichton]

The bots are failing due to opaque rpath issues. I believe this was caused by b20e748 in #32252 (cc @durka)

I'm tired and gonna go to bed, but I've got a build running which I'll hopefully be able to diagnose tomorrow morning. Just wanted to file this to make sure we know nightlies aren't working.

[durka]

Sucks. That change was necessary to make c-dynamic-dylib pass on my test server (host x86-64, target x86-unknown-linux). Are we just stuck backing out the whole thing and having #[derive(Hash)] broken until rustbuild works?