[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