[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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186336 91177308-0d34-0410-b5e6-96231b3b80d8