[PECOFF] Move files with ".lib" extension to the end of the input file list.

It's allowed to specify library files *before* object files in the command
line. Object files seems to be processed first, and then their undefined
symbols are resolved from the libraries. This patch implements the compatible
behavior.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195295 91177308-0d34-0410-b5e6-96231b3b80d8