[llvm-objcopy][MachO] Recompute and update offset/size fields in the writer

Summary:
Recompute and update offset/size fields so that we can implement llvm-objcopy options like --only-section.

This patch is the first step and focuses on supporting load commands that covered by existing tests: executable files and
dynamic libraries are not supported.

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap, rupprecht

Subscribers: compnerd, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62652

llvm-svn: 362863