[PECOFF] Use ulittle32_t::operator+=.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217639 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Do r217639 leftover.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217645 91177308-0d34-0410-b5e6-96231b3b80d8