Cranelift: Add LLVM IR as a target code

I know that LLVM IR is not really similar with real machine isa, but there's some advantages if we can easily translate Cretonne IR to LLVM IR.

It's well-known that LLVM is awfully slow for "bad" code, while cretonne is fast-by-default but currently lacking deep optimizations. I saw that there's a plan to use cretonne for rustc debug mode. But why should compilers handles two different target IR? If we can generate "pretty good" LLVM IR by running cretonne with `--target=llvm`, not just rustc but also whole range of languages whose compiler is written in rust can benefit from it.