Add support for custom targets in the style of xargo/cargo-xbuild

Hello.

I've been using xargo for a while and recently checked out cargo-xbuild. Curious as to how these work, I realised all that you need to do is build the libcore-etc for the target and put these in the sysroot directory. Rustup manages its own sysroot for common targets.

It would be really nice if we could instead do rustup target add x86_64-sel4-robigalia.json to ask rustup to track the target file and rebuild the libcore-etc when either rustc updates or when the user attempts to build something using the target with a newer rustc or when the target file changes.

As a bonus feature it might be faster (or bandwidth-cheaper or just more convenient such as when offline) for some people to build any simple targets using only rust-src and a collection of target.json files.

I'm currently just using unmodified and unwrapped cargo with only these two environment variables set after having used xargo/xbuild at least once after calling rustup update.

export RUSTFLAGS="--sysroot $HOME/my_project/target/sysroot"
export RUSTDOCFLAGS=$RUSTFLAGS

Start by tracing the rustup target add flow and how rustup manages sysroots and rust-src; the issue names no source files or tests, so first identify those entry points. Done means custom target JSON files can be tracked, libcore-etc is rebuilt when rustc or the target changes, and simple targets can optionally use rust-src offline.