[RuntimeDyld] Handle endiannes differences between the host and target while
reading MachO files magic numbers in RuntimeDyld.

This is required now that we're testing cross-platform JITing (via
RuntimeDyldChecker), and should fix some issues that David Fang has seen on PPC
builds.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213012 91177308-0d34-0410-b5e6-96231b3b80d8