bugfix: correct computation of stack size on Mac Posix port

Computes the stack size after the end is aligned to a page
boundary.  The original code in (FreeRTOS#674) did not account for
this and some combinations of stack addresses and sizes
would cause a segfault.

Tested on ARM64 MacOS and Linux, but not Intel (yet).

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