[llvm-objcopy][MachO] Fix strict-aliasing warning. NFCI

Summary:
Use MachOObjectFile::isRelocationScattered instead of reinterpret_cast.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42360

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap

Subscribers: dendibakh, bjope, uabelho, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63699

llvm-svn: 364252