This commit switches the default build system for Rust from the makefiles to rustbuild. The rustbuild build system has been in development for almost a year now and has become quite mature over time. This commit is an implementation of the proposal on [internals] which slates deletion of the makefiles on 2016-01-02. [internals]: https://internals.rust-lang.org/t/proposal-for-promoting-rustbuild-to-official-status/4368 This commit also updates various documentation in `README.md`, `CONTRIBUTING.md`, `src/bootstrap/README.md`, and throughout the source code of rustbuild itself. Closes #37858