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

[gokturk]

Summary

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.

llvm/lib/Support/CMakeLists.txt

if( HAVE_BACKTRACE AND NOT "${Backtrace_LIBRARIES}" STREQUAL "" )
     STRING(REGEX REPLACE "^lib" "" Backtrace_LIBFILE ${Backtrace_LIBFILE})
     set(system_libs ${system_libs} ${Backtrace_LIBFILE})
   endif()
   if(LLVM_ENABLE_TERMINFO)
     if(HAVE_TERMINFO)
       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})
   if( UNIX AND NOT (BEOS OR HAIKU) )
     set(system_libs ${system_libs} m)
   endif()
   if( FUCHSIA )
     set(system_libs ${system_libs} zircon)