stabilize stage management for rustc tools
https://github.com/rust-lang/rust/pull/135990 got out of control due to excessive complexity. This PR aims to achieve the same goal with a simpler approach, likely through multiple smaller PRs. I will keep the other one read-only and open as a reference for future work.
This work stabilizes the staging logic for `ToolRustc` programs, so you no longer need to handle build and target compilers separately in steps. Previously, most tools didn't do this correctly, which was causing the compiler to be built twice (e.g., `x test cargo --stage 1` would compile the stage 2 compiler before, but now it only compiles the stage 1 compiler).
I also tried to document how we should write `ToolRustc` steps as they are quite different and require more attention than other tools.
Next goal is to stabilize how stages are handled for the rustc itself. Currently, `x build --stage 1` builds the stage 1 compiler which is fine, but `x build compiler --stage 1` builds stage 2 compiler.
~~for now, r? ghost~~
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