[libunwind] [libunwind] Add CMake option to enable execute-only code generation on AArch64 (PR #140554)

[Csanád Hajdú via cfe-commits]

Il-Capitano wrote:

https://github.com/llvm/llvm-project/pull/143698 was approved recently, which was the final outstanding PR related to execute-only code generation.

I've rebased this PR, and I'll merge it at the end of the week.

https://github.com/llvm/llvm-project/pull/140554