Move ZST ABI handling to rustc_target

[beetrees]

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

[rustbot]

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[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[bors]

☔ The latest upstream changes (presumably #124733) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

r=me after rebase

[beetrees]

@estebank Rebased. I've also added a normalize-stderr-test to the tests (taken from tests/ui/abi/debug.rs) to prevent the tests being sensitive to preferred alignment differences between targets.

@rustbot label -S-waiting-on-author