[PECOFF] Refactor COFF section header creation.

Code to create COFF section header was scattered across many member functions
of SectionChunk. Consolidate it to a member function of SectionHeaderTableChunk.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196895 91177308-0d34-0410-b5e6-96231b3b80d8