compiler: Clean up weird `rustc_abi` reexports
Just general cleanup in `rustc_target` and `rustc_abi`. I was originally going to make a PR with a larger change that also fixed the last few crates and in doing so removed some clutter from `rustc_abi`, but wound up slightly stuck on it, then figured out how to fix it, and then got distracted by other things... so now I'm trying to figure out what I had figured out earlier.
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