[JITLink] Add initial Aarch64 support

Set up basic infrastructure for 64-bit ARM architecture support in JITLink. It allows for loading a minimal object file and resolving a single relocation. Advanced features like GOT and PLT handling or relaxations were intentionally left out for the moment.

This patch follows the idea to keep implementations for ARM (32-bit) and Aaarch64 (64-bit) separate, because:
* it might be easier to share code with the MachO "arm64" JITLink backend
* LLVM has individual targets for ARM and Aaarch64 as well

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D108986