bpo-40244: Remove XLC's support from the noreturn flag (GH-20588)

Automerge-Triggered-By: @pablogsal
(cherry picked from commit 033d10bd)

Co-authored-by: Batuhan Taskaya <batuhanosmantaskaya@gmail.com>

Include/pyport.h

@@ -829,10 +829,10 @@ extern _invalid_parameter_handler _Py_silent_invalid_parameter_handler;
 #endif
 
 /* Mark a function which cannot return. Example:
+   PyAPI_FUNC(void) _Py_NO_RETURN PyThread_exit_thread(void);
 
-   PyAPI_FUNC(void) _Py_NO_RETURN PyThread_exit_thread(void); */
+   XLC support is intentionally omitted due to bpo-40244 */
 #if defined(__clang__) || \
-    defined(__xlc__) || \
     (defined(__GNUC__) && \
      ((__GNUC__ >= 3) || \
       (__GNUC__ == 2) && (__GNUC_MINOR__ >= 5)))