Fix std_example.rs on s390x

Author: bjorn3

example/std_example.rs

@@ -166,6 +166,7 @@ fn main() {
         enum Never {}
     }
 
+    #[cfg(not(target_arch = "s390x"))] // s390x doesn't have vector instructions enabled by default
     foo(I64X2([0, 0]));
 
     transmute_wide_pointer();