signal/sig_dispatch: Fix case where signal action is sent twice

As far as I can interpret how signal delivery should work when the signal
is blocked, it should still be sent to the pending queue even if the signal
is masked. When the sigmask changes it will be delivered.

The original implementation did not add the pending signal action, if
stcb->task_state == TSTATE_WAIT_SIG is true.

An attempt to patch this was made in #8563 but it is insufficient as it
creates an issue when the task is not waiting for a signal, but is in
syscall, in this case the signal is incorrectly queued twice.

sched/signal/sig_dispatch.c

@@ -372,14 +372,9 @@ int nxsig_tcbdispatch(FAR struct tcb_s *stcb, siginfo_t *info)
   if (masked == 1)
 #endif
     {
-#ifdef CONFIG_LIB_SYSCALL
-      /* If the thread is in syscall, schedule the sigaction here */
+      /* Add signal to pending queue regardless. */
 
-      if ((stcb->flags & TCB_FLAG_SYSCALL) != 0)
-        {
       nxsig_add_pendingsignal(stcb, info);
-        }
-#endif
 
       /* Check if the task is waiting for this pending signal. If so, then
        * unblock it. This must be performed in a critical section because
@@ -411,19 +406,9 @@ int nxsig_tcbdispatch(FAR struct tcb_s *stcb, siginfo_t *info)
             {
               up_switch_context(stcb, rtcb);
             }
-
-          leave_critical_section(flags);
         }
 
-      /* Its not one we are waiting for... Add it to the list of pending
-       * signals.
-       */
-
-      else
-        {
       leave_critical_section(flags);
-          nxsig_add_pendingsignal(stcb, info);
-        }
     }
 
   /************************* UNMASKED SIGNAL ACTIONS ************************/