[cmake] Attempt to fix sanitizer buildbot.
The generation of the native_export_file end up in
several different makefiles. All those makefiles
write the same file, but can be executed concurrently...
and bad things happen!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199356 91177308-0d34-0410-b5e6-96231b3b80d8