Delete thread key on process exit (#1297)

Previously, the shared thread key was deleted in xPortStartScheduler
after scheduler was ended. This created a race condition where
prvThreadKeyDestructor (responsible for freeing thread-specific heap
memory) would not be called if xPortStartScheduler deleted the key
before the last task deletion, as destructors are not invoked after key
deletion (see https://github.com/walac/glibc/blob/master/nptl/pthread_create.c#L145-L150).

Move thread key deletion to process exit to ensure all thread-specific
memory is properly freed.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

portable/ThirdParty/GCC/Posix/port.c

@@ -140,6 +140,8 @@ static void prvThreadKeyDestructor( void * pvData )
 static void prvInitThreadKey( void )
 {
     pthread_key_create( &xThreadKey, prvThreadKeyDestructor );
+    /* Destroy xThreadKey when the process exits. */
+    atexit( prvDestroyThreadKey );
 }
 /*-----------------------------------------------------------*/
 
@@ -315,8 +317,6 @@ BaseType_t xPortStartScheduler( void )
     /* Restore original signal mask. */
     ( void ) pthread_sigmask( SIG_SETMASK, &xSchedulerOriginalSignalMask, NULL );
 
-    prvDestroyThreadKey();
-
     return 0;
 }
 /*-----------------------------------------------------------*/