bpo-34170, bpo-34207: Fix pymain_init_cmdline_argv()

* Add _Py_DecodeUTF8()
* pymain_init_cmdline_argv() now calls directly _Py_DecodeUTF8() if
  the UTF-8 mode is enabled

Previously, pymain_init_cmdline_argv() always called
Py_DecodeLocale() which depends on Py_UTF8Mode, whereas this variable
is no longer modified when reading the current configuration.