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

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: [e56b479](https://github.com/llvm/llvm-project/commit/e56b479d9f096171287e23169ce23295046535c0)

https://github.com/llvm/llvm-project/actions/runs/17808821563

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

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