[llvm-objcopy][MachO] Implement a layout algorithm for executables

Summary: The layout algorithm for relocatable objects and for executable are somewhat different. This patch implements the latter one based on the algorithm in LLD (MachOFileLayout).

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap

Subscribers: jakehehrlich, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65539

llvm-svn: 369231