Link rustllvm statically to librustc

In order to keep up to date with changes to the libraries that `llvm-config`
spits out, the dependencies to the LLVM are a dynamically generated rust file.
This file is now automatically updated whenever LLVM is updated to get kept
up-to-date.

At the same time, this cleans out some old cruft which isn't necessary in the
makefiles in terms of dependencies.

Closes #10745
Distribute a unified binary for snapshots

This means that a snapshot is purely a binary with no rust dynamic dependencies!

Closes #10744
auto merge of #10809 : alexcrichton/rust/static-snapshot, r=brson

Now that we have the necessary logic in rustc for windows, this is possible to land.