process: use more direct sync I/O for stdio

This avoids routing writes through the full LibuvStreamWrap
write machinery. In particular, it enables the next commit,
because otherwise the callback passed to `_write()`
would not be called synchronously for pipes on Windows
(because the latter does not support `uv_try_write()`,
even for blocking I/O).

PR-URL: https://github.com/nodejs/node/pull/18019


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>