[libunwind] Add CMake option to enable execute-only code generation on AArch64 (#140554)
For a full toolchain supporting execute-only code generation the runtime
libraries also need to be pre-compiled with it enabled. The generic
`RUNTIMES_EXECUTE_ONLY_CODE` CMake option can now be used during build
configuration to enable execute-only code generation in libunwind.
Related RFC:
https://discourse.llvm.org/t/rfc-execute-only-code-support-for-runtime-libraries-on-aarch64/86180