Make sure libLLVM users link with libatomic if needed

64-bit atomics are used in llvm/ADT/Statistic.h, which means that users
of libLLVM.so might also have to link with libatomic. To avoid having
to special-case the library here, we simply add all `LLVM_SYSTEM_LIBS`
as public link dependencies to libLLVM.

This fixes a build failure on PowerPC 32-bit.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D132799

llvm/tools/llvm-shlib/CMakeLists.txt

@@ -63,6 +63,10 @@ if(LLVM_BUILD_LLVM_DYLIB)
 
   target_link_libraries(LLVM PRIVATE ${LIB_NAMES})
 
+  if(LLVM_ENABLE_THREADS AND NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+    target_link_libraries(LLVM PUBLIC atomic)
+  endif()
+
   if (APPLE)
     set_property(TARGET LLVM APPEND_STRING PROPERTY
                 LINK_FLAGS