Retire the legacy `Makefile`-based `run-make` test infra The final piece of [porting run-make tests to use Rust #121876](https://github.com/rust-lang/rust/issues/121876). Closes #121876. Closes #40713. Closes #81791 (no longer using `wc`). Closes #56475 (no longer a problem in current form of that test; we don't ignore the test on `aarch64-unknown-linux-gnu`). ### Summary This PR removes the legacy `Makefile`-based `run-make` test infra which has served us well over the years. The legacy infra is no longer needed since we ported all of `Makefile`-based `run-make` tests to the new `rmake.rs` infra. Additionally, this PR: - Removes `tests/run-make/tools.mk` since no more `Makefile`-based tests remain. - Updates `tests/run-make/README.md` and rustc-dev-guide docs to remove mention about `Makefile`-based `run-make` tests - Update test suite requirements in rustc-dev-guide on Windows to no longer need MSYS2 (they should also now run successfully on native Windows MSVC). - Update `triagebot.toml` to stop backlinking to #121876. **Thanks to everyone who helped in this effort to modernize the `run-make` test infra and test suite!** r? bootstrap