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_15~++20220517092516+a09af8669396/libcxxabi/src/CMakeLists.txt
===================================================================
--- llvm-toolchain-snapshot_15~++20220517092516+a09af8669396.orig/libcxxabi/src/CMakeLists.txt
+++ llvm-toolchain-snapshot_15~++20220517092516+a09af8669396/libcxxabi/src/CMakeLists.txt
@@ -77,7 +77,9 @@ else()
   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)