Add the start of the support for llvm-objdump’s -private-headers for Mach-O files.

This adds the printing of the mach header. Load command printing will be next.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216285 91177308-0d34-0410-b5e6-96231b3b80d8