bootstrap: Allow building individual crates

This aims to be as unintrusive as possible, but did still require adding a new `tail_args` field to all `Rustc` and `Std` steps.

New library and compiler crates are added to the sysroot as they are built, since it's useful to have e.g. just alloc and not std.

Fixes https://github.com/rust-lang/rust/issues/44293.