[JITLink][AArch64] Lift fixup functions from aarch64.cpp to aarch64.h. (NFC)

Lift fixup functions from aarch64.cpp to aarch64.h so that they have better chance of getting inlined. Also, adds some comments documenting the purpose of functions.

Reviewed By: sgraenitz

Differential Revision: https://reviews.llvm.org/D127559