[llvm-objcopy][MachO] Rebuild the symbol/string table in the writer

Summary: Build the string table using StringTableBuilder, reassign symbol indices, and update symbol indices in relocations to allow adding/modifying/removing symbols from the object.

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap

Subscribers: mgorny, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63309

llvm-svn: 364000