[ORC][LLJIT] Use JITLink by default on ELF/x86-64.

This patch switches LLJIT's default JIT linker for ELF/x86-64 from RuntimeDyld
to JITLink.

Most clients should not be affected, but if you were explicitly accessing the
old RTDyldObjectLinkingLayer (e.g. to install JITEventListeners) you will need
to either force use of RuntimeDyld (following the example in
llvm/examples/OrcV2Examples/LLJITWithCustomObjectLinkingLayer), or switch to
using JITLink plugins instead.