gdbserver add attach support

components/lwp/arch/arm/cortex-a/lwp_gcc.S

@@ -183,7 +183,12 @@ ret_to_user:
     cmp r0, #0
     pop {r0-r3, r12, lr}
     bne user_do_signal
-
+#ifdef RT_USING_GDBSERVER
+    push {r0-r3, r12, lr}
+    mov r0, lr
+    bl lwp_check_debug_attach_req
+    pop {r0-r3, r12, lr}
+#endif
     movs pc, lr
 
 #ifdef RT_USING_LWP

include/rtdef.h

@@ -744,10 +744,13 @@ struct rt_thread
     struct rt_wakeup wakeup;                            /**< wakeup data */
     int exit_request;
 #ifdef RT_USING_USERSPACE
+#ifdef RT_USING_GDBSERVER
     int step_exec;
+    int debug_attach_req;
     int debug_ret_user;
     int debug_suspend;
     struct rt_hw_exp_stack *regs;
+#endif
     void * thread_idr;                                 /** lwp thread indicator */
     int *clear_child_tid;
 #endif