[ORC] Always use ObjectLinkingLayer/JITLink for MachO on x86-64 and arm64.

JITLink supports all code and relocation models, so there's no reason to
conditionalize using JITLink on the code or relocation model settings.

Clients wanting to use RTDyldObjectLinkingLayer/RuntimeDyld will now
need to use a custom object linking layer creator.