[Runtimes][CMake] Add CMake option to enable execute-only code generation on AArch64

[Il-Capitano]

Based on the discussion in https://discourse.llvm.org/t/rfc-execute-only-code-support-for-runtime-libraries-on-aarch64/86180 a single, global configuration option should be used to enable execute-only code generation for the runtime libraries. The new option LLVM_EXECUTE_ONLY_CODE adds the -mexecute-only flag to CMAKE_C_FLAGS and CMAKE_CXX_FLAGS, which simplifies the library-level configuration needed for the runtime libraries.

Project-specific changes are still needed to handle assembly sources, e.g. in compiler-rt and libunwind.

[Il-Capitano]

@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.

[petrhosek]

@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 #134893 which introduces RUNTIMES_USE_LIBC).

[Il-Capitano]

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.

[Il-Capitano]

Gentle ping.

I would like to get this approved and merged by the end of the month, before the LLVM 21 release. Can you review, or add other relevant reviewers, please?

[Il-Capitano]

Ping

[Il-Capitano]

Ping @ldionne @petrhosek