[yaml2obj] Support for dumping mach_header from yaml

With this change obj2yaml and yaml2obj can now round-trip mach_headers.

This change also adds ObjectYAML/MachO tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269314 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed