Fix omrthread_get_stack_range on Mac OSX

On Mac OSX stack start means where sp begins not the low memory.
This is the opposite of how it is on linux and how this API is
intended to be used.

Signed-off-by: Tobi Ajila <atobia@ca.ibm.com>

thread/common/omrthreadinspect.c

@@ -352,9 +352,9 @@ omrthread_get_stack_range(omrthread_t thread, void **stackStart, void **stackEnd
 	OSTHREAD osTid = thread->handle;
 	size_t stackSize = 0;
 
-	*stackStart = pthread_get_stackaddr_np(osTid);
+	*stackEnd = pthread_get_stackaddr_np(osTid);
 	stackSize = pthread_get_stacksize_np(osTid);
-	*stackEnd = (void *)((uintptr_t)*stackStart + stackSize);
+	*stackStart = (void *)((uintptr_t)*stackEnd - stackSize);
 	return J9THREAD_SUCCESS;
 #else /* defined(OSX) */
 	return J9THREAD_ERR_UNSUPPORTED_PLAT;