x64: Add LJ_GC64 mode interpreter.

Enable this mode with: make XCFLAGS=-DLUAJIT_ENABLE_GC64

src/lj_cdata.c

@@ -121,7 +121,12 @@ collect_attrib:
     idx = (ptrdiff_t)intV(key);
     goto integer_key;
   } else if (tvisnum(key)) {  /* Numeric key. */
-    idx = LJ_64 ? (ptrdiff_t)numV(key) : (ptrdiff_t)lj_num2int(numV(key));
+#ifdef _MSC_VER
+    /* Workaround for MSVC bug. */
+    volatile
+#endif
+    lua_Number n = numV(key);
+    idx = LJ_64 ? (ptrdiff_t)n : (ptrdiff_t)lj_num2int(n);
   integer_key:
     if (ctype_ispointer(ct->info)) {
       CTSize sz = lj_ctype_size(cts, ctype_cid(ct->info));  /* Element size. */