[mach-o] Add support for upward linking

To deal with cycles in shared library dependencies, the darwin linker supports
marking specific link dependencies as "upward".  An upward link is when a
lower level library links against a higher level library.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219949 91177308-0d34-0410-b5e6-96231b3b80d8