Extract librustc_back from librustc
rustc: Invert some rpath dependencies
Extract rpath to rustc_back::rpath
rustc: Invert some archive deps
rustc: Move ArchiveRO to rustc_llvm

It is a wrapper around LLVM.
rustc: Move archive to rustc_back
rustc_back: Update crate docs

Indicate that anything that can be extracted here should and that things with
syntax deps should be split out someday.
rustc: Move util::fs to rustc_back
rustc: Move util::sha2 to rustc_back