[PECOFF] Add the entry point file at the right place.

The entry point file needs to be processed after all other
object files and before all .lib files. It was processed
after .lib files. That caused an issue that the entry point
function was not resolved from the standard library files.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213804 91177308-0d34-0410-b5e6-96231b3b80d8