x64/LJ_GC64: Add missing backend support and enable JIT compilation.

Contributed by Peter Cawley.

src/lj_record.c

@@ -976,7 +976,13 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
     }
     /* The cdata metatable is treated as immutable. */
     if (LJ_HASFFI && tref_iscdata(ix->tab)) goto immutable_mt;
+#if LJ_GC64
+    /* TODO: fix ARM32 asm_fload(), so we can use this for all archs. */
+    ix->mt = mix.tab = lj_ir_ggfload(J, IRT_TAB,
+      GG_OFS(g.gcroot[GCROOT_BASEMT+itypemap(&ix->tabv)]));
+#else
     ix->mt = mix.tab = lj_ir_ktab(J, mt);
+#endif
     goto nocheck;
   }
   ix->mt = mt ? mix.tab : TREF_NIL;