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

[via llvm-commits]

aankit-ca wrote:

@androm3da This change should not affect non musl environments and I've verified it. The CCState is initialized with TreatAsVarArg which is defined like this

```
  // Linux ABI treats var-arg calls the same way as regular ones.
  bool TreatAsVarArg = !Subtarget.isEnvironmentMusl() && IsVarArg;
```

At the moment, this TreatAsVarArg is ignored. The State.isVarArg() should check for this value after this change.

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