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

[Fangrui Song via llvm-commits]

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+# RUN: llvm-mc %s -triple=xtensa -show-encoding --mattr=+windowed \
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MaskRay wrote:

Yes, you might want to port the tool to Xtensa and then add a test to `llvm/test/tools/UpdateTestChecks/update_mc_test_checks/`

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