Dynamic LLVM

[brson]

Distro's generally want to use their own system LLVM. Rust's support for this has traditionally been spotty, but we currently do CI builds with a statically-linked 3.7. There's demand for using dynamic linking for LLVM as well (Gentoo), so we'll get that working and set up CI for it too.

We'll guarantee in the future that Rust always builds with some recent release of LLVM.

See the existing PR.

re https://internals.rust-lang.org/t/perfecting-rust-packaging-the-plan/2767

[brson]

This is done. Thanks @DiamondLovesYou