bugfix: correct computation of stack size on Mac Posix port

Aligns the stack end to a page boundary before computing its
size, since the size depends on both the start and end.

The original change which introduced stack alignment (FreeRTOS#674)
only worked for cases where the round + trunc operation would
wind up within the same area, but would lead to segfaults in
other cases.

Tested on ARM64 and Intel MacOS, as well as ARM64 and Intel
Linux.  The test cases included a single-task case, as well
as a case with two tasks passing queue messages.

portable/ThirdParty/GCC/Posix/port.c

@@ -148,10 +148,14 @@ StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
      */
     thread = ( Thread_t * ) ( pxTopOfStack + 1 ) - 1;
     pxTopOfStack = ( StackType_t * ) thread - 1;
+
+    #ifdef __APPLE__
+        pxEndOfStack = ( StackType_t *) mach_vm_round_page( pxEndOfStack );
+    #endif
+
     ulStackSize = ( size_t ) ( pxTopOfStack + 1 - pxEndOfStack ) * sizeof( *pxTopOfStack );
 
     #ifdef __APPLE__
-        pxEndOfStack = mach_vm_round_page( pxEndOfStack );
         ulStackSize = mach_vm_trunc_page( ulStackSize );
     #endif