bugfix: correct computation of stack size on Mac Posix port (#816)

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 (#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.

Also adds a typecast to the `mach_vm_round_page()` call, as
it is actually a macro which casts to `mach_vm_offset_t` and
the result here is used as a `StackType_t` pointer.

Tested on ARM64 and Intel MacOS, as well as ARM64 and Intel
Linux.  The test code 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