On ELF and COFF treat linker_private like private.

The linkers on these systems don't have anything special to do with these
symbols. Since the intent is for them to be absent from the final object,
just treat them as private.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197080 91177308-0d34-0410-b5e6-96231b3b80d8