MachObjectWriter: optimize the string table for common suffices

This is a follow-up to r207670 (ELF) and r218636 (COFF).

Differential Revision: http://reviews.llvm.org/D5622

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219126 91177308-0d34-0410-b5e6-96231b3b80d8