Register new snapshots and tweak the snapshot process

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.