if LIBCXXABI_USE_COMPILER_RT=ON, add builtins to libcxxabi library
flags, otherwise add libgcc following how libunwind does the same
Index: llvm-toolchain-13-13.0.1~+rc1/libcxxabi/src/CMakeLists.txt
===================================================================
--- llvm-toolchain-13-13.0.1~+rc1.orig/libcxxabi/src/CMakeLists.txt
+++ llvm-toolchain-13-13.0.1~+rc1/libcxxabi/src/CMakeLists.txt
@@ -111,7 +111,9 @@ if (ANDROID AND ANDROID_PLATFORM_LEVEL L
   list(APPEND LIBCXXABI_LIBRARIES android_support)
 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 ()