[PECOFF][Writer] Structure of pe32_header was changed.

Change for http://llvm-reviews.chandlerc.com/D952

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D953

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183789 91177308-0d34-0410-b5e6-96231b3b80d8