[Support] Check for atomics64 when deciding if '-latomic' is needed

The CheckAtomic module performs two tests to determine if passing
'-latomic' to the linker is required: one for 64-bit atomics, and
another for non-64-bit atomics. Include the missing check for 64-bit
atomics.

Reviewers: beanz, compnerd
Reviewed By: beanz, compnerd
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69444

Author: gktrk

llvm/lib/Support/CMakeLists.txt

@@ -26,7 +26,7 @@ elseif( CMAKE_HOST_UNIX )
       set(system_libs ${system_libs} ${TERMINFO_LIBS})
     endif()
   endif()
-  if( LLVM_ENABLE_THREADS AND HAVE_LIBATOMIC )
+  if( LLVM_ENABLE_THREADS AND (HAVE_LIBATOMIC OR HAVE_CXX_LIBATOMICS64) )
     set(system_libs ${system_libs} atomic)
   endif()
   set(system_libs ${system_libs} ${LLVM_PTHREAD_LIB})