LJ_GC64: Return true for ffi.abi("gc64").

doc/ext_ffi_api.html

@@ -468,6 +468,8 @@ otherwise. The following parameters are currently defined:
 <td class="abiparam">eabi</td><td class="abidesc">EABI variant of the standard ABI</td></tr>
 <tr class="odd">
 <td class="abiparam">win</td><td class="abidesc">Windows variant of the standard ABI</td></tr>
+<tr class="even">
+<td class="abiparam">gc64</td><td class="abidesc">64 bit GC references</td></tr>
 </table>
 
 <h3 id="ffi_os"><tt>ffi.os</tt></h3>

src/lib_ffi.c

@@ -751,6 +751,9 @@ LJLIB_CF(ffi_abi)	LJLIB_REC(.)
   case H_(4ab624a8,4ab624a8): b = 1; break;  /* win */
 #endif
   case H_(3af93066,1f001464): b = 1; break;  /* le/be */
+#if LJ_GC64
+  case H_(9e89d2c9,13c83c92): b = 1; break;  /* gc64 */
+#endif
   default:
     break;
   }