Add back parsing of header charactestics.

It had been dropped during the switch to yaml::IO. Also add a test going
from yaml2obj to llvm-readobj. It can be extended as we add more
fields/formats to yaml2obj.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178786 91177308-0d34-0410-b5e6-96231b3b80d8