MachO: allow each section to have a linker-private symbol

MachO: allow each section to have a linker-private symbol

The upcoming ARM64 backend doesn't have section-relative relocations,
so we give each section its own symbol to provide this functionality.
Of course, it doesn't need to appear in the final executable, so
linker-private is the best kind for this purpose.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205081 91177308-0d34-0410-b5e6-96231b3b80d8