[PECOFF] Another Win64 relocation bug fix

When a relocation is applied to a location, the new value needs
to be added to the existing value at the location. Existing
value is in most cases zero, but if not, the current code does
not work.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@216680 91177308-0d34-0410-b5e6-96231b3b80d8