Set CXX_<target> in bootstrap

[ids1024]

I came across this trying to cross-compile rustc for Redox. It was also mentioned in a comment on #42206, but doesn't seem to have been corrected.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Thanks for the PR! I think though that this'll need to be conditional as accessing the cxx method can panic for non-host targets. Mind either changing cxx to return an Option or have a similar check?

[ids1024]

I've made it return a Result, since unwrapping an Option wouldn't provide a useful error message.

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit f98ffb5 has been approved by alexcrichton

[Mark-Simulacrum]

@bors rollup