[llvm-objcopy][MachO] Implement --only-section

Reviewers: alexshap, rupprecht, jdoerfert, jhenderson

Reviewed By: alexshap, rupprecht, jhenderson

Subscribers: mgorny, jakehehrlich, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65541