libgcc: properly destroy mutexes on VxWorks

Just as one needs run-time initialization of mutexes, one needs to
destroy them properly to allow the OS to release resources associated
with the semaphore.

From-SVN: r265616

libgcc/ChangeLog

+2018-10-30  Rasmus Villemoes  <rv@rasmusvillemoes.dk>
+
+	* config/gthr-vxworks.h (__gthread_mutex_destroy): Call semDelete.
+
 2018-10-25  Martin Liska  <mliska@suse.cz>
 
 	PR other/87735

libgcc/config/gthr-vxworks.h

@@ -63,8 +63,9 @@ __gthread_mutex_init_function (__gthread_mutex_t *mutex)
 }
 
 static inline int
-__gthread_mutex_destroy (__gthread_mutex_t * UNUSED(mutex))
+__gthread_mutex_destroy (__gthread_mutex_t *mutex)
 {
+  semDelete(*mutex);
   return 0;
 }