[obj2yaml] Support ELF input format in the obj2yaml tool.

The ELF header e_flags field in the MIPS related test cases handled
incorrectly. The obj2yaml prints too many flags. I will fix that in the
next patches.

The patch reviewed by Michael Spencer and Sean Silva.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208752 91177308-0d34-0410-b5e6-96231b3b80d8