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

[Petr Hosek via llvm-commits]

petrhosek wrote:

> @ldionne In a comment on #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.

That's because we don't yet have a runtimes-specific variables but we expect to have more soon (see for example https://github.com/llvm/llvm-project/pull/134893 which introduces `RUNTIMES_USE_LIBC`).

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