Explain what `Assemble` does in bootstrap

[jyn514]

I am not actually sure what this does. It can't just be copying files because I tried running the binary in stage0-rustc and it gave an error about shared objects:

build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/rustc-main: error while loading shared libraries: librustc_driver-3b0ece280f85df92.so: cannot open shared object file: No such file or directory

Originally posted by @jyn514 in #857 (comment)

[Mark-Simulacrum]

It just copies files. You get linker errors otherwise because cargo directory layout doesn't match our rpath (../lib iirc)