[yaml2obj] Add support for ELF e_flags.

Summary:
The FileHeader mapping now accepts an optional Flags sequence that accepts
the EF_<arch>_<flag> constants. When not given, Flags defaults to zero.

Reviewers: atanasyan

Reviewed By: atanasyan

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3213

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205173 91177308-0d34-0410-b5e6-96231b3b80d8