Don't use a git dep for ar

https://github.com/bjorn3/rustc_codegen_cranelift/pull/1201 changed the Cranelift deps to crates.io releases.

There is still a git dependency on my fork of the ar crate (https://github.com/bjorn3/rust-ar/tree/do_not_remove_cg_clif_ranlib). I think I will need to fix symbol table support for macOS and then poke the maintainer of rust-ar to release a new version of my PR.