rustc_trans: ignore trailing padding larger than 8 bytes.

src/test/run-pass/align-with-extern-c-fn.rs

@@ -14,10 +14,10 @@
 #![feature(attr_literals)]
 
 #[repr(align(16))]
-pub struct A {
-    y: i64,
-}
+pub struct A(i64);
 
 pub extern "C" fn foo(x: A) {}
 
-fn main() {}
+fn main() {
+    foo(A(0));
+}