Factor librustc_target from rustc into a standalone library

[sunfishcode]

librustc_target is a library in rustc for implementing a lot of native ABI and toolchain logic. It has some overlap with target-lexicon, however it provides much more extensive features, especially including knowledge of calling conventions. It's written in a way which is mostly independent from the rest of rustc, so it's an interesting candidate for factoring out into a standalone library that Cranelift users and others could use to more easily integrate with native C ABI/toolchain environments.

This aligns fairly well with cranelift-codegen's rough design for calling conventions, in which it only does the lowest-level parts, and assumes that cranelift-frontend or other libraries will be the place for offering higher-level functionality such as handling struct types.

See this comment for some more details on librustc_target.

This library isn't Cranelift-specific, as other projects could make use of such a library too. That said, it is particularly interesting for use with Cranelift, so I'm posting an issue here so we can track it.