Kernel: Make addresses returned by kmalloc() properly aligned for x86_64

Kernel/Heap/Heap.h

@@ -20,6 +20,10 @@ class Heap {
 
     struct AllocationHeader {
         size_t allocation_size_in_chunks;
+#if ARCH(X86_64)
+        // FIXME: Get rid of this somehow
+        size_t alignment_dummy;
+#endif
         u8 data[0];
     };