Move ZST ABI handling to `rustc_target`
Currently, target specific handling of ZST function call ABI (specifically passing them indirectly instead of ignoring them) is handled in `rustc_ty_utils`, whereas all other target specific function call ABI handling is located in `rustc_target`. This PR moves the ZST handling to `rustc_target` so that all the target-specific function call ABI handling is in one place. In the process of doing so, this PR fixes#125850 by ensuring that ZST arguments are always correctly ignored in the x86-64 `"sysv64"` ABI; any code which would be affected by this fix would have ICEd before this PR. Tests are also added using `#[rustc_abi(debug)]` to ensure this behaviour does not regress.
Fixes#125850
d0293c6cf2