[PowerPC] Correctly use ELFv2 ABI on all OS's that use the ELFv2 ABI

Add a member function isPPC64ELFv2ABI() to determine what ABI is used on the
64-bit PowerPC big endian operating environment.

Reviewed By: nemanjai, dim, pkubaj

Differential Revision: https://reviews.llvm.org/D144321