Fix asmconstants for win64 (LLP64)

asmconstants.h

@@ -1,10 +1,30 @@
-#ifdef __LP64__
+#if defined(__SIZEOF_POINTER__) && defined(__SIZEOF_LONG__)
+#define LIBOBJC2_POINTER_WIDTH __SIZEOF_POINTER__
+#define LIBOBJC2_LONG_WIDTH __SIZEOF_LONG__
+#elif defined(__LP64__)
+#define LIBOBJC2_POINTER_WIDTH 8
+#define LIBOBJC2_LONG_WIDTH 8
+#else // defined(__LP64__)
+#define LIBOBJC2_POINTER_WIDTH 4
+#define LIBOBJC2_LONG_WIDTH 4
+#endif // SIZEOF_POINTER/LONG, __LP64__
+
+#if LIBOBJC2_POINTER_WIDTH == 8 && LIBOBJC2_LONG_WIDTH == 8
+// 64-bit (non-windows)
 #define DTABLE_OFFSET  64
 #define SMALLOBJ_BITS  3
 #define SHIFT_OFFSET   0
 #define DATA_OFFSET    8
 #define SLOT_OFFSET    32
+#elif LIBOBJC2_POINTER_WIDTH == 8 && LIBOBJC2_LONG_WIDTH == 4
+// 64-bit (windows)
+#define DTABLE_OFFSET  56
+#define SMALLOBJ_BITS  3
+#define SHIFT_OFFSET   0
+#define DATA_OFFSET    8
+#define SLOT_OFFSET    32
 #else
+// 32-bit (all other targets)
 #define DTABLE_OFFSET  32
 #define SMALLOBJ_BITS  1
 #define SHIFT_OFFSET   0