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

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
`RUNTIMES_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.

runtimes/CMakeLists.txt

@@ -233,6 +233,20 @@ endif()
 option(LLVM_INCLUDE_TESTS "Generate build targets for the runtimes unit tests." ON)
 option(LLVM_INCLUDE_DOCS "Generate build targets for the runtimes documentation." ON)
 option(LLVM_ENABLE_SPHINX "Use Sphinx to generate the runtimes documentation." OFF)
+option(RUNTIMES_EXECUTE_ONLY_CODE "Compile runtime libraries as execute-only." OFF)
+
+if (RUNTIMES_EXECUTE_ONLY_CODE)
+  # If a target doesn't support or recognise -mexecute-only, Clang will simply ignore the flag.
+  # We can check for this case using -Werror=unused-command-line-argument.
+  check_c_compiler_flag("-mexecute-only -Werror=unused-command-line-argument" C_SUPPORTS_MEXECUTE_ONLY)
+  if (NOT C_SUPPORTS_MEXECUTE_ONLY)
+    message(FATAL_ERROR "RUNTIMES_EXECUTE_ONLY_CODE was turned on, but the target '${LLVM_TARGET_TRIPLE}'"
+                        " doesn't support the -mexecute-only flag")
+  endif()
+
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -mexecute-only")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mexecute-only")
+endif()
 
 # Use libtool instead of ar if you are both on an Apple host, and targeting Apple.
 if(CMAKE_HOST_APPLE AND APPLE)