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