the support for PSE51 (#5534)

* [add] the function realization of signal for posix.
* [update] the posix support for armclang.
* [add] the new macro "RT_USING_POSIX_TIMER".
* [modify] select "RT_USING_SOFT_TIMER" when use posix'timer.
* [bug] optimize the logic for the "time_xxx" functions.
* [modify] use "RT_USING_POSIX_TIMER" to protect the macro definition.
* [modify] error code when except happened.
* [delete] the "environ" is useless at this time.

src/timer.c

@@ -581,6 +581,8 @@ rt_err_t rt_timer_control(rt_timer_t timer, int cmd, void *arg)
             /*timer is stop*/
             *(rt_uint32_t *)arg = RT_TIMER_FLAG_DEACTIVATED;
         }
+    case RT_TIMER_CTRL_GET_REMAIN_TIME:
+        *(rt_tick_t *)arg =  timer->timeout_tick;
         break;
 
     default: