Fix ObjCPass on big-endian host

Summary:
This fixes test/mach-o/objc-image-info-pass-output.yaml on
big-endian hosts.

Reviewers: lhames, kledzik, ruiu

Reviewed By: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35052

llvm-svn: 308935