[Object] Use BFD name for little-endian PowerPC64

Summary:
Little-endian PowerPC object files should report "elf64-powerpcle" instead of
"elf64-powerpc".

Reviewers: jhenderson, MaskRay, espindola, alexshap, rupprecht, #powerpc

Reviewed By: MaskRay

Subscribers: wuzish, emaste, nemanjai, shchenz, steven.zhang, llvm-commits

Tags: #llvm, #powerpc

Differential Revision: https://reviews.llvm.org/D78344