kernel: add CODE_UNREACHABLE in _StackCheckHandler

* _StackCheckHandler is FUNC_NORETURN
* if _ARCH_EXCPET is redefined for specific arch and
  has function return in some cases, e.g., interrupt or
  exception, a compiler warning will come out
* So add CODE_UNREACHABLE to guarantee it will not return

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>

kernel/compiler_stack_protect.c

@@ -36,6 +36,7 @@ void FUNC_NORETURN _StackCheckHandler(void)
 	/* Stack canary error is a software fatal condition; treat it as such.
 	 */
 	_k_except_reason(_NANO_ERR_STACK_CHK_FAIL);
+	CODE_UNREACHABLE;
 }
 
 /* Global variable */