trans: Specify archive_format for MSVC

This means that we no longer need to ship the `llvm-ar.exe` binary in the MSVC
distribution, and after a snapshot we can remove a good bit of logic from the
makefiles!

src/librustc_back/target/windows_msvc_base.rs

@@ -60,6 +60,7 @@ pub fn opts() -> TargetOptions {
             "/NOLOGO".to_string(),
             "/NXCOMPAT".to_string(),
         ],
+        archive_format: "gnu".to_string(),
 
         .. Default::default()
     }