[libunwind] [libunwind] Support extended amd64 segment registers (PR #229176)

[Minsoo Choo via cfe-commits]

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mchoo7 wrote:

arm64ec (Windows arm64) can run x86 binary, that's where the issue comes from. arm64ec lets running arm64 and amd64 code in one process by mapping ABI (thus register slots). Our libunwind doesn't have support for these registers although they do exist on Windows.

https://github.com/llvm/llvm-project/pull/229176