switching to gnu archive format per review request

Author: Dave Huseby

src/librustc_back/target/bitrig_base.rs

@@ -20,6 +20,7 @@ pub fn opts() -> TargetOptions {
         linker_is_gnu: true,
         has_rpath: true,
         position_independent_executables: true,
+        archive_format: "gnu".to_string(),
 
         .. Default::default()
     }