LibJS: Align HeapBlock cell storage to __BIGGEST_ALIGNMENT__

alignas(Cell) is just the alignment requirement for the base class
(which resolves to 8-bytes). This does not necessarily hold for all
subclasses.

Userland/Libraries/LibJS/Heap/HeapBlock.h

@@ -111,7 +111,7 @@ private:
     size_t m_cell_size { 0 };
     size_t m_next_lazy_freelist_index { 0 };
     GCPtr<FreelistEntry> m_freelist;
-    alignas(Cell) u8 m_storage[];
+    alignas(__BIGGEST_ALIGNMENT__) u8 m_storage[];
 
 public:
     static constexpr size_t min_possible_cell_size = sizeof(FreelistEntry);