[llvm] [Xtensa] Implement Windowed Register Option. (PR #124656)

[Andrei Safronov via llvm-commits]

andreisfr wrote:

Previous PR which implemented  "WIndowed register option" https://github.com/llvm/llvm-project/pull/121118 was reviewed and merged, but then reverted because there was a link error to checkRegister function from AsmParser and Disassembler modules.  In this variant "checkRegister" function placed in XtensaMCTargetDesc.

https://github.com/llvm/llvm-project/pull/124656