[JITLink] Unify x86-64 MachO and ELF 's optimize GOT/Stub function

This patch  unify optimizeELF_x86_64_GOTAndStubs and optimizeMachO_x86_64_GOTAndStubs into a pure optimize_x86_64_GOTAndStubs

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D108025