[libunwind] [libunwind][NFC] Remove the CET keyword in shadow stack-related stuffs (PR #126663)

[John Brawn via cfe-commits]

john-brawn-arm wrote:

These functions aren't exposed in any of the header files in libunwind/include so I don't think renaming them should be a problem. Renaming things to use architecture-neutral terms sounds like a good idea to me.

https://github.com/llvm/llvm-project/pull/126663