switching to gnu archive format

Author: Dave Huseby

src/librustc_back/target/freebsd_base.rs

@@ -18,7 +18,7 @@ pub fn opts() -> TargetOptions {
         executables: true,
         morestack: true,
         has_rpath: true,
-        archive_format: "".to_string(),
+        archive_format: "gnu".to_string(),
 
         .. Default::default()
     }