[JITLink] Initial implementation of ELF / x86-64 support for JITLink.

This initial implementation supports section and symbol parsing, but no
relocation support. It enables JITLink to link and execute ELF relocatable
objects that do not require relocations.

Patch by Jared Wyles. Thanks Jared!

Differential Revision: https://reviews.llvm.org/D79832