[PECOFF][Writer] Implement the writer that can emit text section.

lld can now output a valid Windows executable with a text section that does nothing
but just returns immediately. It's not able to handle relocations, symbol tables,
data sections, etc, so it still can't do anything practical, though.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D892