GH-108819: fix LIBDEST not honoring --with-platlibdir (#133163)

GH-108819: fix LIBDEST not honoring --with-platlibdir (#133163)

* GH-108819: fix LIBDEST not honoring --with-platlibdir

We look for the pure-Python part of the standard library in
PLATSTDLIBDIR, which may not match the default LIBDIR subdir.

From ``getpath.py``:

```python
    ...
    STDLIB_SUBDIR = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}{ABI_THREAD}'
    STDLIB_LANDMARKS = [f'{STDLIB_SUBDIR}/os.py', f'{STDLIB_SUBDIR}/os.pyc']
    PLATSTDLIB_LANDMARK = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}{ABI_THREAD}/lib-dynload'
    ...
```

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Add news

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Always set LIBDEST and BINLIBDEST based on PLATLIBDIR

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Add XXX comment on PLATLIBDIR default value

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Regen configure

Signed-off-by: Filipe Laíns <lains@riseup.net>
---------

Signed-off-by: Filipe Laíns <lains@riseup.net>