Add function "rop_write_momery_gadget"
 * Delete "rop_search_gadgets", it's not feasible enough.
 * Add "rop_write_momery_gadget"
 * to find "mov [a] b; pop a; pop b; xor b, b" gadget.
 * However, there are something need to be improved.
 *  -> if some write gadgets not find, return tree_search
 *  -> long gadget
 * Update function "rop_chain_list_add" can add head.