Easing the setup of `compiler_builtins`

[ojeda]

What would it take to make compiler_builtins be something that didn't need to manually be installed? There are a few approaches:

• For rustup to include it as a component, so that installation is easier (and it is automatically updated etc.).
• That the rust repo includes it (moonshot, but for those that need rust-src it is much more likely that they need it too).
• Removing it as a dependency by copying the needed builtins into the tree (the kernel does this for others, too).
• Removing it as a dependency by not using alloc (e.g. if for other reasons we end up with our own alloc).