[MCJIT] Move endian-aware read/writes from RuntimeDyldMachO into
RuntimeDyldImpl.

These are platform independent, and moving them to the base class allows
RuntimeDyldChecker to use them too.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216801 91177308-0d34-0410-b5e6-96231b3b80d8