[JITLink][ELF][ppc64] Add skeleton ppc64 support and ELF/ppc64 JITLink backend.

This patch introduces a skeleton JITLink ppc64 support header and ELF/ppc64
backend. No relocations are supported in this initial version, but given a
program requiring no relocations (e.g. one that just returns a constant value
from main) the new backend is able to construct a LinkGraph from a ppc64 ELF
relocatable object, and the llvm-jitlink tool is able to execute it.

This commit should also serve as a good example of how to introduce a JITLink
backend for a new architecture.

Reviewed By: sgraenitz, v.g.vassilev, vchuravy, nemanjai, jain98, MaskRay

Differential Revision: https://reviews.llvm.org/D148192