rustbuild: allow using ninja to build LLVM

[nagisa]

Ninja is much faster at building LLVM, especially when more cores get involved. We should either:

1. Autodetect presence of ninja and use it automatically (with an option to force use of make); or
2. Allow using ninja (via some option).

[alexcrichton]

If anyone wants to dive into rustbuild, this should be a pretty easy bug to tackle! (in theory).

You'd probably want to start out by adding a check to sanity.rs to just see if ninja exists and then when we later build LLVM you should just be able to pass the argument to CMake!

We already just call cmake --build so I think that takes care of running ninja when it's configured, so we just need to tell CMake to use ninja.

[caipre]

I'll take this.