Rollup merge of #149967 - folkertdev:va-list-hexagon, r=workingjubilee

custom `VaList` layout for Hexagon

I noticed while browsing LLVM source that we use an incorrect `VaList` definition for the musl hexagon target.

relevant links

- https://github.com/llvm/llvm-project/blob/0cdc1b6dd4a870fc41d4b15ad97e0001882aba58/clang/include/clang/Basic/TargetInfo.h#L333
- https://github.com/llvm/llvm-project/blob/0cdc1b6dd4a870fc41d4b15ad97e0001882aba58/clang/lib/CodeGen/Targets/Hexagon.cpp#L407-L417

cc target maintainer `@androm3da` can you confirm that this looks OK? In particular the `#[rustc_pass_indirectly_in_non_rustic_abis]` attribute is used to simulate pointer decay (like if the struct were wrapped in a 1-element array in C). The clang comment suggests that the Tag is wrapped in such a single-element array, but I haven't actually been able to confirm it.

For stabilizing `c_variadic` (on the hexagon targets) we will also need a custom `va_arg` implementation to mirror the one in `clang` in [va_arg.rs](https://github.com/rust-lang/rust/blob/main/compiler/rustc_codegen_llvm/src/va_arg.rs). Would you be able to contribute one?

r? `@workingjubilee`

Author: JonathanBrouwer