RuntimeDyld should use the memory manager API.

Start teaching the runtime Dyld interface to use the memory manager API
for allocating space. Rather than mapping directly into the MachO object,
we extract the payload for each object and copy it into a dedicated buffer
allocated via the memory manager. For now, just do Segment64, so this works
on x86_64, but not yet on ARM.

llvm-svn: 128973