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.

components/libc/posix/pthreads/pthread.h

@@ -185,6 +185,10 @@ int pthread_setschedprio(pthread_t thread, int prio);
 void pthread_exit (void *value_ptr);
 int pthread_once(pthread_once_t * once_control, void (*init_routine) (void));
 
+#ifdef RT_USING_SIGNALS
+int pthread_sigmask(int how, const sigset_t *set, sigset_t *oset);
+#endif
+
 /* pthread cleanup */
 void pthread_cleanup_pop(int execute);
 void pthread_cleanup_push(void (*routine)(void*), void *arg);