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

[via cfe-commits]

jinge90 wrote:

> Normally we should not remove symbols for backward compatibility, but in this niche case I am not sure anyone uses these CET function names, so I think the rename is fine. Instead of `ss`, perhaps `shstk`? I agree that it's useful to unify the naming on aarch64/riscv/x86 if they do end up being very similar.
> 
> @john-brawn-arm @jinge90

It seems to be OK to rename since these functions are not intended to be called by users, they are just used internally in libunwind.

Thanks very much.

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