Fix problem in metadata writer

It was creating non-multiple-of-four section sizes, which, for some
reason, presumably by LLVM, were clipped, rather than padded, to be a
multiple of four.

src/comp/middle/metadata.rs

@@ -589,6 +589,10 @@ fn create_index[T](vec[tup(T, uint)] index, fn(&T) -> uint hash_fn)
     encode_index[int](ebml_w, items_buckets, int_writer);
     ebml.end_tag(ebml_w);
 
+    // Pad this, since something (LLVM, presumably) is cutting off the
+    // remaining % 4 bytes.
+    buf_w.write(vec(0u8, 0u8, 0u8, 0u8));
+
     ret C_postr(string_w.get_str());
 }