[MCJIT] Significantly refactor the RuntimeDyldMachO class.

The previous implementation of RuntimeDyldMachO mixed logic for all targets
within a single class, creating problems for readability, maintainability, and
performance. To address these issues, this patch strips the RuntimeDyldMachO
class down to just target-independent functionality, and moves all
target-specific functionality into target-specific subclasses RuntimeDyldMachO.

The new class hierarchy is as follows:

class RuntimeDyldMachO
Implemented in RuntimeDyldMachO.{h,cpp}
Contains logic that is completely independent of the target. This consists
mostly of MachO helper utilities which the derived classes use to get their
work done.


template <typename Impl>
class RuntimeDyldMachOCRTPBase<Impl> : public RuntimeDyldMachO
Implemented in RuntimeDyldMachO.h
Contains generic MachO algorithms/data structures that defer to the Impl class
for target-specific behaviors.

RuntimeDyldMachOARM : public RuntimeDyldMachOCRTPBase<RuntimeDyldMachOARM>
RuntimeDyldMachOARM64 : public RuntimeDyldMachOCRTPBase<RuntimeDyldMachOARM64>
RuntimeDyldMachOI386 : public RuntimeDyldMachOCRTPBase<RuntimeDyldMachOI386>
RuntimeDyldMachOX86_64 : public RuntimeDyldMachOCRTPBase<RuntimeDyldMachOX86_64>
Implemented in their respective *.h files in lib/ExecutionEngine/RuntimeDyld/MachOTargets
Each of these contains the relocation logic specific to their target architecture.
In Process::LoadImage, if dlopen returns 0x0 fetch the error with dlerror and report
that in the returned Error.