gpython: Fix thinko when rejecting unknown -X option

Before:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ gpython -X gpython.zzz
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/z-dev/bin/gpython", line 3, in <module>
        from gpython import main; main()
      File "/home/kirr/src/tools/go/pygolang/gpython/__init__.py", line 397, in main
        raise RuntimeError('gpython: unknown -X option %s' % opt)
    RuntimeError: gpython: unknown -X option -X                                        <-- NOTE

After:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ gpython -X gpython.zzz
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/z-dev/bin/gpython", line 3, in <module>
        from gpython import main; main()
      File "/home/kirr/src/tools/go/pygolang/gpython/__init__.py", line 397, in main
        raise RuntimeError('gpython: unknown -X option %s' % arg)
    RuntimeError: gpython: unknown -X option gpython.zzz                               <-- NOTE

/proposed-for-review-on nexedi/pygolang!25

gpython/__init__.py

@@ -395,7 +395,7 @@ def main():
                     sys._xoptions['gpython.runtime'] = gpy_runtime
 
                 else:
-                    raise RuntimeError('gpython: unknown -X option %s' % opt)
+                    raise RuntimeError('gpython: unknown -X option %s' % arg)
 
                 continue