Rollup merge of #117671 - kjetilkjeka:nvptx_c_abi_avoid_direct, r=davidtwco

NVPTX: Avoid PassMode::Direct for args in C abi

Fixes #117480

I must admit that I'm confused about `PassMode` altogether, is there a good sum-up threads for this anywhere? I'm especially confused about how "indirect" and "byval" goes together. To me it seems like "indirect" basically means "use a indirection through a pointer", while "byval" basically means "do not use indirection through a pointer".

The return used to keep `PassMode::Direct` for small aggregates. It turns out that `make_indirect` messes up the tests and one way to fix it is to keep `PassMode::Direct` for all aggregates. I have mostly seen this PassMode mentioned for args. Is it also a problem for returns? When experimenting with `byval` as an alternative i ran into [this assert](https://github.com/rust-lang/rust/blob/61a3eea8043cc1c7a09c2adda884e27ffa8a1172/compiler/rustc_codegen_llvm/src/abi.rs#L463C22-L463C22)

I have added tests for the same kind of types that is already tested for the "ptx-kernel" abi. The tests cannot be enabled until something like #117458 is completed and merged.

CC: ``@RalfJung`` since you seem to be the expert on this and have already helped me out tremendously

CC: ``@RDambrosio016`` in case this influence your work on `rustc_codegen_nvvm`

``@rustbot`` label +O-NVPTX

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