Add LJ_GC64 mode: 64 bit GC object references.

Actually NaN tagging with 47 bit pointers and 13+4 bit tags.

src/lj_jit.h

@@ -213,6 +213,9 @@ typedef struct GCtrace {
   uint8_t topslot;	/* Top stack slot already checked to be allocated. */
   uint8_t linktype;	/* Type of link. */
   IRRef nins;		/* Next IR instruction. Biased with REF_BIAS. */
+#if LJ_GC64
+  uint32_t unused_gc64;
+#endif
   GCRef gclist;
   IRIns *ir;		/* IR instructions/constants. Biased with REF_BIAS. */
   IRRef nk;		/* Lowest IR constant. Biased with REF_BIAS. */