[lld][PECOFF] Fix data directory entry RVA of base relocations section

Summary:
This patch changes WriterPECOFF to actually write down the address instead of ignoring it.
Also, it changes the order of adding the BaseReloc chunk as otherwise the address wasn't set yet.

I think a better way of doing it would be to change DataDirectoryAtom to create a Reference
instead of using a number, and to change IdataPass accordingly, but I'm not sure how to do that.

Reviewers: ruiu

Reviewed By: ruiu

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1743