[llvm] [Xtensa] Implement Windowed Register Option. (PR #121118)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave commented:

It seems like you only implement the MC support (and no codegen support) for windowed registers, could you put that in either your title and/or description?

https://github.com/llvm/llvm-project/pull/121118