Add ffi.abi("dualnum").

Thanks to Sergey Kaplun. #1442

doc/ext_ffi_api.html

@@ -468,6 +468,8 @@ otherwise. The following parameters are currently defined:
 <td class="abiparam">uwp</td><td class="abidesc">Universal Windows Platform</td></tr>
 <tr class="even">
 <td class="abiparam">gc64</td><td class="abidesc">64 bit GC references</td></tr>
+<tr class="odd">
+<td class="abiparam">dualnum</td><td class="abidesc">Dual-number mode</td></tr>
 </table>
 
 <h3 id="ffi_os"><tt>ffi.os</tt></h3>

src/lib_ffi.c

@@ -758,6 +758,9 @@ LJLIB_CF(ffi_abi)	LJLIB_REC(.)
 #endif
 #if LJ_GC64
     "\004gc64"
+#endif
+#if LJ_DUALNUM
+    "\007dualnum"
 #endif
   ) >= 0;
   setboolV(L->top-1, b);