Align start of the page's object area to match OS::kMaxPreferredCodeAlignment.

runtime/vm/pages.h

Index: runtime/vm/pages.h
diff --git a/runtime/vm/pages.h b/runtime/vm/pages.h
index f3e67a36dfb2df95b38dfd3bfa1a46b297b65ac3..0d4bb02ed29facf87895e4b8e2dff57ea1ed47f8 100644
--- a/runtime/vm/pages.h
+++ b/runtime/vm/pages.h
@@ -36,7 +36,7 @@ class HeapPage {
 
   uword object_start() const {
     return (reinterpret_cast<uword>(this) +
-            Utils::RoundUp(sizeof(HeapPage), kObjectAlignment));
+            Utils::RoundUp(sizeof(HeapPage), OS::kMaxPreferredCodeAlignment));
   }
   uword object_end() const {
     return object_end_;