Create a common include that can be used by everything that depends on the Rust source. Including stage0 bits and hashes of the source tarballs.
There were two different do_install_append()'s and they didn't check for errors so this improves that situation.
Convert rust-llvm to not use the shared source setup and instead use its own extracted directory. Include the version info in the bitbake file so that in the future we can support multiple versions via a PREFERRED_VERSIONS variable.