Fix omrthread_get_stack_range behaviour on AIX

On AIX 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

@@ -335,8 +335,18 @@ omrthread_get_stack_range(omrthread_t thread, void **stackStart, void **stackEnd
 #endif /* (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) */
 	pthread_attr_destroy(&attr);
 
+#if defined(AIXPPC)
+	/* On AIX 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.
+	 */
+	*stackEnd = *stackStart;
+	*stackStart = (void *)((uintptr_t)*stackEnd - stackSize);
+#else /* defined(AIXPPC) */
 	/* On Linux, native stack grows from high to low memory */
 	*stackEnd = (void *)((uintptr_t)*stackStart + stackSize);
+#endif /* defined(AIXPPC) */
+
 	return J9THREAD_SUCCESS;
 #elif defined(OSX) /* defined(LINUX) || defined(AIXPPC) */
 	OSTHREAD osTid = thread->handle;