[JITLink][PowerPC][RFC] Make JITLink default in LLJIT for ppc64 elfv2abi

JITLink for ppc64 has implemented all relocations implemented in rtdyld for ppc64 and has passed all existed lit tests and unittests in llvm/clang/compiler-rt. I propose making JITLink for ppc64 default in LLJIT.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D159175