[llvm] [Runtimes][CMake] Add CMake option to enable execute-only code generation on AArch64 (PR #143698)

[Csanád Hajdú via llvm-commits]

Il-Capitano wrote:

@ldionne
In a comment on https://github.com/llvm/llvm-project/pull/140555 @petrhosek suggested I change the CMake option's name to `RUNTIMES_EXECUTE_ONLY_CODE`. You also suggested this name previously on the discourse RFC.
Should I make that change? My main hold-up on making that change is that I don't see any other CMake option following that naming convention in current main, but if this is the convention that should be used going forward I'm happy to make that change.

https://github.com/llvm/llvm-project/pull/143698