rustbuild feature to not rebuild a lower stage

[est31]

Often you are in a situation where you want to try some modifications of some basic library like libcore or some other library with the stage1 compiler, without rebuilding stage1.

A param like --assume-stage=n passed to rustbuild which doesn't (re-)build stage n artifacts would be really neat for this.

@bluss had the original idea, and it would be really useful for development on the i128 PR as well: there, large parts of the libcompiler_builtins crate are under #[cfg(not(stage0))], so I'm only interested in what happens in higher stages when modifying them.