[obj2yaml] - Support PPC64 relocation types.

We do not support them and fail with llvm_unreachable currently.
This is not the only target we do not support and also seems we are missing
the tests for those we have already. But I needed this one for another patch,
so posted it separatelly.

Relocation names are taken from llvm\include\llvm\BinaryFormat\ELFRelocs\PowerPC64.def

Differential revision: https://reviews.llvm.org/D67615

llvm-svn: 372109