sysconfig values for `platstdlib` do not appear to be accurate

Bug report

Bug description:

The values set for platstdlib in the sysconfig scheme paths are not consistent with the way sys.path is configured in getpath.py

sysconfig: https://github.com/python/cpython/blob/main/Lib/sysconfig/__init__.py#L28
getpath: https://github.com/python/cpython/blob/main/Modules/getpath.py#L182

Given this appears to have been wrong essentially forever, it appears nobody is using platstdlib for anything important (even I only noticed because running -m sysconfig inside a venv reports a platstdlib path inside the venv, which is definitely not right).

Still, it should be possible to make them at least a bit more consistent with the way sys.path is actually initialised (appending lib-dynload to the stdlib path looks to be sufficient for non-Windows system, but I'm less sure on Windows)

CPython versions tested on:

3.12

Operating systems tested on:

Linux, Windows

Lib/sysconfig/init.py#L28 と Modules/getpath.py#L182 を比較し、まず Linux と Windows で platstdlib がどのように形成されるかを確認する。仮想環境の内外で sys.path と python -m sysconfig を使って結果のパスを検証する；完了の条件は、両方のプラットフォームで platstdlib が実際の初期化を反映していることである。