Recognize 0x0000 as a COFF file magic.

Summary:
Some machine-type-neutral object files containing only undefined symbols
actually do exist in the Windows standard library. Need to recognize them
as COFF files.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2164

llvm-svn: 194734