Fetch compiler-builtins for targets where it isn't already included by rustup

[WildCryptoFox]

When doing any development using xargo or cargo-xbuild with a custom target, the compiler-builtins crate needs to be fetched. This used to just work. shrug

[alexcrichton]

This crate is now included natively in Cargo as of rust-lang/cargo#6869 and today's nightly Rust. I'm closing all issues in this repository before I archive the repository, but if you're still interested in seeing this issue fixed then please feel free to file an issue with Cargo itself!