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"``,