Auto merge of #37817 - alexcrichton:rustbuild-default, r=brson

mk: Switch rustbuild to the default build system

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
2017-02-02.

[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.