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

[Csanád Hajdú via llvm-commits]

Il-Capitano wrote:

> That's because we don't yet have a runtimes-specific variables but we expect to have more soon (see for example #134893 which introduces `RUNTIMES_USE_LIBC`).

Thanks for the explanation, I updated the name.

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