Register new snapshots
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 #10747 : alexcrichton/rust/snapshots, r=cmr

This registers new snapshots after the landing of #10528, and then goes on to tweak the build process to build a monolithic `rustc` binary for use in future snapshots. This mainly involved dropping the dynamic dependency on `librustllvm`, so that's now built as a static library (with a dynamically generated rust file listing LLVM dependencies).

This currently doesn't actually make the snapshot any smaller (24MB => 23MB), but I noticed that the executable has 11MB of metadata so once progress is made on #10740 we should have a much smaller snapshot.

There's not really a super-compelling reason to distribute just a binary because we have all the infrastructure for dealing with a directory structure, but to me it seems "more correct" that a snapshot compiler is just a `rustc` binary.