Allow ObjectFilePECOFF to initialize with ARM binaries.

Summary: This is required to start debugging WinPhone ARM targets.

Reviewers: compnerd, zturner, omjavaid

Reviewed By: compnerd

Subscribers: jasonmolenda, aemerson, rengolin, lldb-commits

Differential Revision: https://reviews.llvm.org/D19604

llvm-svn: 316532