[PECOFF] Support .lib archive file.

Archive file in Windows has file extension of ".lib" but the file format is
in fact the same as Unix. It's an ar archive holding multiple .obj files.
The existing archive reader can read .lib files.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184036 91177308-0d34-0410-b5e6-96231b3b80d8