fix linkage tests

Author: Manishearth

src/test/compile-fail/linkage1.rs

@@ -11,4 +11,5 @@
 extern {
     #[linkage = "extern_weak"] static foo: isize;
     //~^ ERROR: the `linkage` attribute is experimental and not portable
+    //~^^ ERROR: the `linkage` attribute is experimental and not portable
 }

src/test/compile-fail/linkage4.rs

@@ -10,6 +10,6 @@
 
 #[linkage = "external"]
 static foo: isize = 0;
-//~^ ERROR: the `linkage` attribute is experimental and not portable
+//~^^ ERROR: the `linkage` attribute is experimental and not portable
 
 fn main() {}