[llvm] [Hexagon] Bugfix in VarArg lowering: Special case for musl (PR #157564)

[via llvm-commits]

llvmbot wrote:

<!--IGNORE-->
>/cherry-pick [e56b479](https://github.com/llvm/llvm-project/commit/e56b479d9f096171287e23169ce23295046535c0)

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/157564