Set ninja=true by default
Ninja substantially improves LLVM build time. On a 96-way system, using
Make took 248s, and using Ninja took 161s, a 35% improvement.
We already require a variety of tools to build Rust. If someone wants to
build without Ninja (for instance, to minimize the set of packages
required to bootstrap a new target), they can easily set `ninja=false`
in `config.toml`. Our defaults should help people build Rust (and LLVM)
faster, to speed up development.
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