[PECOFF][Writer] Replace magic numbers with sizeof().

This is a follow up patch for r186336.

Reviewers: LegalizeAdulthood

CC: silvas, llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1144

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186384 91177308-0d34-0410-b5e6-96231b3b80d8