Re-enable trace stitching.

Thanks to Vyacheslav Egorov.

src/lj_jit.h

@@ -381,6 +381,7 @@ typedef struct jit_State {
   GCRef *trace;		/* Array of traces. */
   TraceNo freetrace;	/* Start of scan for next free trace. */
   MSize sizetrace;	/* Size of trace array. */
+  TValue *ktracep;	/* Pointer to K64Array slot with GCtrace pointer. */
 
   IRRef1 chain[IR__MAX];  /* IR instruction skip-list chain anchors. */
   TRef slot[LJ_MAX_JSLOTS+LJ_STACK_EXTRA];  /* Stack slot map. */