[PECOFF] Resource file extension is .res, not .rc.

This patch won't change LLD's behavior because it's a temporary file and
LLD does not use the file extension to determine file type. But using the
correct file extension is a good thing.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193211 91177308-0d34-0410-b5e6-96231b3b80d8