[libunwind] Add CMake option to enable execute-only code generation on AArch64 (llvm#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

Author: Il-Capitano

libunwind/CMakeLists.txt

@@ -332,6 +332,10 @@ if (C_SUPPORTS_COMMENT_LIB_PRAGMA)
   endif()
 endif()
 
+if (RUNTIMES_EXECUTE_ONLY_CODE)
+  add_compile_definitions(_LIBUNWIND_EXECUTE_ONLY_CODE)
+endif()
+
 #===============================================================================
 # Setup Source Code
 #===============================================================================

libunwind/src/UnwindRegistersRestore.S

@@ -18,6 +18,8 @@
 
 #if defined(_AIX)
   .toc
+#elif defined(__aarch64__) && defined(__ELF__) && defined(_LIBUNWIND_EXECUTE_ONLY_CODE)
+  .section .text,"axy",@progbits,unique,0
 #else
   .text
 #endif

libunwind/src/UnwindRegistersSave.S

@@ -18,6 +18,8 @@
 
 #if defined(_AIX)
     .toc
+#elif defined(__aarch64__) && defined(__ELF__) && defined(_LIBUNWIND_EXECUTE_ONLY_CODE)
+    .section .text,"axy",@progbits,unique,0
 #else
     .text
 #endif