Rationalize what is public in RuntimeDyldMachO and RuntimeDyldELF.

The implemented RuntimeDyldImpl interface is public. Everything else is private.
Since these classes are not inherited from (yet), there is no need to have
protected members.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180733 91177308-0d34-0410-b5e6-96231b3b80d8