POSIX Port: Round up stack sizes correctly on macOS

On macOS, pthread_attr_setstacksize requires the stack size to be a multiple of
the system page size.

The previous changes here assumed the use of pthread_attr_setstack, however
the transition to pthread_attr_setstacksize means we can avoid rounding the
pxEndOfStack pointer.

This is additionally positive because pxEndOfStack was actually being rounded
in the wrong direction -- it ought to have been rounded down (_trunc) instead of
up due to the stack growth direction. In my case this actually caused pxEndOfStack
to end up after pxTopOfStack, which underflowed and created a huge stack size
request + EXC_BAD_ACCESS in pthread_create.

Signed-off-by: Paul Hollinsky <paulhollinsky@gmail.com>

portable/ThirdParty/GCC/Posix/port.c

@@ -165,14 +165,14 @@ 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__
-        ulStackSize = mach_vm_trunc_page( ulStackSize );
+        /*
+         * On macOS, pthread_attr_setstacksize requires the stack to be a multiple of the system page size.
+         * Round up to the next page boundary.
+         */
+        ulStackSize = mach_vm_round_page( ulStackSize );
     #endif
 
     thread->pxCode = pxCode;