if libatomic is found only link it if libgcc is the rtlib
Index: llvm-toolchain-12_12.0.1/libcxx/CMakeLists.txt
===================================================================
--- llvm-toolchain-12_12.0.1.orig/libcxx/CMakeLists.txt
+++ llvm-toolchain-12_12.0.1/libcxx/CMakeLists.txt
@@ -756,12 +756,14 @@ function(cxx_link_system_libraries targe
     endif()
   elseif (LIBCXX_HAS_GCC_LIB)
     target_link_libraries(${target} PRIVATE gcc)
+    if (LIBCXX_HAS_ATOMIC_LIB)
+      target_link_libraries(${target} PRIVATE atomic)
+    endif()
   elseif (LIBCXX_HAS_GCC_S_LIB)
     target_link_libraries(${target} PRIVATE gcc_s)
-  endif()
-
-  if (LIBCXX_HAS_ATOMIC_LIB)
-    target_link_libraries(${target} PRIVATE atomic)
+    if (LIBCXX_HAS_ATOMIC_LIB)
+      target_link_libraries(${target} PRIVATE atomic)
+    endif()
   endif()
 
   if (MINGW)