c-variadic: make `va_arg` match on `Arch` exhaustive
tracking issue: https://github.com/rust-lang/rust/issues/44930
Continuing from https://github.com/rust-lang/rust/pull/150094, the more annoying cases remain. These are mostly very niche targets without Clang `va_arg` implementations, and so it might just be easier to defer to LLVM instead of us getting the ABI subtly wrong. That does mean we cannot stabilize c-variadic on those targets I think.
Alternatively we could ask target maintainers to contribute an implementation. I'd honestly prefer they make that change to LVM though (likely by just using `CodeGen::emitVoidPtrVAArg`) that we can mirror.
r? @workingjubilee