bpo-31904: Add encoding support for VxWorks RTOS (GH-12051)

Use UTF-8 as the system encoding on VxWorks.

The main reason are:

1. The locale is frequently misconfigured.
2. Missing some functions to deal with locale in VxWorks C library.
diff --git a/Doc/c-api/sys.rst b/Doc/c-api/sys.rst
index 0eee35a..c8ea783 100644
--- a/Doc/c-api/sys.rst
+++ b/Doc/c-api/sys.rst
@@ -108,7 +108,7 @@
 
    Encoding, highest priority to lowest priority:
 
-   * ``UTF-8`` on macOS and Android;
+   * ``UTF-8`` on macOS, Android, and VxWorks;
    * ``UTF-8`` on Windows if :c:data:`Py_LegacyWindowsFSEncodingFlag` is zero;
    * ``UTF-8`` if the Python UTF-8 mode is enabled;
    * ``ASCII`` if the ``LC_CTYPE`` locale is ``"C"``,
@@ -154,7 +154,7 @@
 
    Encoding, highest priority to lowest priority:
 
-   * ``UTF-8`` on macOS and Android;
+   * ``UTF-8`` on macOS, Android, and VxWorks;
    * ``UTF-8`` on Windows if :c:data:`Py_LegacyWindowsFSEncodingFlag` is zero;
    * ``UTF-8`` if the Python UTF-8 mode is enabled;
    * ``ASCII`` if the ``LC_CTYPE`` locale is ``"C"``,
diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index 39c067d..97c5ebc 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -760,8 +760,8 @@
                                                         Py_ssize_t len, \
                                                         const char *errors)
 
-   Decode a string from UTF-8 on Android, or from the current locale encoding
-   on other platforms. The supported
+   Decode a string from UTF-8 on Android and VxWorks, or from the current
+   locale encoding on other platforms. The supported
    error handlers are ``"strict"`` and ``"surrogateescape"``
    (:pep:`383`). The decoder uses ``"strict"`` error handler if
    *errors* is ``NULL``.  *str* must end with a null character but
@@ -796,8 +796,8 @@
 
 .. c:function:: PyObject* PyUnicode_EncodeLocale(PyObject *unicode, const char *errors)
 
-   Encode a Unicode object to UTF-8 on Android, or to the current locale
-   encoding on other platforms. The
+   Encode a Unicode object to UTF-8 on Android and VxWorks, or to the current
+   locale encoding on other platforms. The
    supported error handlers are ``"strict"`` and ``"surrogateescape"``
    (:pep:`383`). The encoder uses ``"strict"`` error handler if
    *errors* is ``NULL``. Return a :class:`bytes` object. *unicode* cannot