if LIBCXXABI_USE_COMPILER_RT=ON, add builtins to libcxxabi library
flags, otherwise add libgcc following how libunwind does the same
Index: llvm-toolchain-snapshot_19~++20240220124703+f122268c0441/libcxxabi/src/CMakeLists.txt
===================================================================
--- llvm-toolchain-snapshot_19~++20240220124703+f122268c0441.orig/libcxxabi/src/CMakeLists.txt
+++ llvm-toolchain-snapshot_19~++20240220124703+f122268c0441/libcxxabi/src/CMakeLists.txt
@@ -75,7 +75,9 @@ if (NOT APPLE) # On Apple platforms, we
   add_library_flags_if(LIBCXXABI_HAS_C_LIB c)
 endif()
 
-if (NOT LIBCXXABI_USE_COMPILER_RT)
+if (LIBCXXABI_USE_COMPILER_RT)
+  add_library_flags("${LIBCXXABI_BUILTINS_LIBRARY}")
+else()
   add_library_flags_if(LIBCXXABI_HAS_GCC_LIB gcc)
 endif()
 if (NOT LIBCXXABI_USE_LLVM_UNWINDER)