I think bootstrap with OCaml is ok - if just bootstrap from source becomes possible at all. Binary-only dependency is awful when Rust is wanted on yet unsupported systems.
The key word here is 'start'. I don't like to arbitrarily constrain tools, but I've always felt that cross-compilers were sort of a last resort, ie, you don't have a native compiler (you're writing the compiler or operating system), or are really pressed for time. Once you've got a native compiler, that should be it: self-hosted from there on.