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

[Csanád Hajdú via cfe-commits]

Il-Capitano wrote:

@MaskRay @ldionne @arichardson
Gentle ping.

I would like to get this approved and merged by the end of the month, can you take a look, please?

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