[PECOFF][Writer] Emit .reloc section.

Emit .reloc section. This is the first step to support DLL creation. The
executable doesn't need .reloc section, but the DLL does.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1126

llvm-svn: 186336