Globally install custom target definition

I'm currently working on making it possible to compile Rust code for the Nintendo Switch with a homebrew toolchain called libtransistor. As part of this work, I have made a custom target definition that, among other things, defines a link script that needs to run.

Using the toolchain is fairly simple : people need to download a forked rust source that contains some fixes for our target, and point xargo at it when compiling their code (to get libcore and liballoc, potentially libstd in the future). Target-specific functions are provided through a cargo dependency. The only problem is, people need to keep around the custom target definition and the associated linker script in every project they have, which is very cumbersome.

It'd be nice if we had a global place, e.g. ~/.cargo/custom-targets, where we could store both the target json and its associated files (like link scripts).

Start by examining how custom target definitions and associated linker scripts are currently supplied to xargo, using the issue's ~/.cargo/custom-targets proposal as the desired entry point. Done means a global location can provide the target JSON and related files without keeping copies in every project.