[JITLink][ELF][AArch64] Lift MachO/arm64 edges into aarch64.h, reuse for ELF.

This patch moves the aarch64 fixup logic from the MachO/arm64 backend to
aarch64.h header so that it can be re-used in the ELF/aarch64 backend. This
significantly expands relocation support in the ELF/aarch64 backend.

Reviewed By: lhames, sgraenitz

Differential Revision: https://reviews.llvm.org/D126286