[llvm] [SPARC] Fix va_arg of long double on the 32-bit ABI (PR #228199)

[Folkert de Vries via llvm-commits]

folkertdev wrote:

Why do you need/want this functionality? 

LLVM`s `va_arg` is very likely to miscompile code, because it does not have sufficient layout information. A frontend must, in effect, implement `va_arg`  itself, much like clang and rustc already do. So I don't think it's really worth spending the lines to implement this logic without a specific use case.  

https://github.com/llvm/llvm-project/pull/228199