test ABI compatibility for some unsized types as well
and test for what `DispatchFromDyn` needs.
Also I ran this on a whole bunch of targets via Miri and added enough `cfg` to make it all work, as documentation for what does and doesn't currently work. (Most of those targets do not have their tests run on CI anyway.)
Here's the shell rune I used for that:
```
for TARGET in x86_64-unknown-linux-gnu x86_64-pc-windows-gnu aarch64-unknown-linux-gnu s390x-unknown-linux-gnu mips64-unknown-linux-gnuabi64 sparc64-unknown-linux-gnu powerpc64-unknown-linux-gnu powerpc64le-unknown-linux-gnu riscv64gc-unknown-linux-gnu loongarch64-unknown-linux-gnu wasm32-unknown-unknown; do
BOOTSTRAP_SKIP_TARGET_SANITY=1 ./x.py run miri --stage 0 --args tests/ui/abi/compatibility.rs --target $TARGET;
done
```
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