compiler: Directly use rustc_abi almost everywhere
Use rustc_abi instead of rustc_target where applicable. This is mostly described by the following substitutions:
```rust
match path_substring {
rustc_target::spec::abi::Abi => rustc_abi::ExternAbi,
rustc_target::abi::call => rustc_target::callconv,
rustc_target::abi => rustc_abi,
}
```
A number of spot-fixes make that not quite the whole story.
The main exception is in 33edc68 where I get a lot more persnickety about how things are imported, especially in `rustc_middle::ty::layout`, not just from where. This includes putting an end to a reexport of `rustc_middle::ty::ReprOptions`, for the same reason that the rest of this change is happening: reexports mostly confound things.
This notably omits rustc_passes and the ast crates, as I'm still examining a question I have about how they do stability checking of `extern "Abi"` strings and if I can simplify their logic. The rustc_abi and rustc_target crates also go untouched because they will be entangled in that cleanup.
r? compiler-errors
72df7780dd