Add `ToolTarget` to bootstrap Oh, you thought I'm done with refactoring bootstrap tools? Na-ah, think again! After the failure of https://github.com/rust-lang/rust/pull/143581, `ToolTarget` is back with a vengeance. This time, I implemented the test changes and tool cleanups without forcing these tools to be built with the stage0 compiler. There are still some small wins though, `LlvmBitcodeLinker` now starts at stage 1, and not stage 2. Cargo should also be ported to this new mode, but I'm leaving that for a follow-up PR. Hopefully X-th time's the charm :crossed_fingers: r? `@jieyouxu`