fix: make QEMU-VExpress-A9 example compatible with newlib 12.x by han…

[loremmoqi]

…dling removed __sdidinit

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

在使用新版本的 newlib 时，RT-Thread 的 QEMU-VExpress-A9 示例在编译过程中出现错误：error: 'struct _reent' has no member named '__sdidinit'。这是因为新版 newlib 移除了 __sdidinit 成员，导致 RT-Thread 的 stdio.c 代码不兼容。本 PR 修复了这一问题，确保示例在新版 newlib 下能够正常编译。

你的解决方案是什么 (what is your solution)

在 components/libc/posix/io/stdio.c 中，添加了条件编译宏以适配 newlib 的版本差异：

#if defined(_REENT_SMALL) || !defined(__sdidinit)
// newlib 新版本没有 __sdidinit
#else
_GLOBAL_REENT->__sdidinit = 1;
#endif
此修改确保在新版 newlib 下，标准输入输出初始化代码能够正确编译，同时保持对旧版 newlib 的兼容性。

请提供验证的bsp和config (provide the config and bsp)

• BSP:bsp/qemu-vexpress-a9

• .config:无需修改 .config 文件，仅涉及代码逻辑的条件编译适配

• action:https://github.com/loremmoqi/rt-thread/blob/master/.github/workflows/manual_dist.yml

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ loremmoqi
❌ github-actions[bot]
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[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

---

🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
   点击进入工作流 → | Click to open workflow →

2. 点击 Run workflow | Click Run workflow

• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")
• 将目标分支设置为 \ Set the target branch to：fix-bug-newlib
• 设置PR number为 \ Set the PR number to：10785

3. 等待工作流完成 | Wait for the workflow to complete
   格式化后的代码将自动推送至你的分支。
   The formatted code will be automatically pushed to your branch.

完成后，提交将自动更新至 fix-bug-newlib 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the fix-bug-newlib branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: components

Reviewers: @Maihuanyi

Changed Files (Click to expand)

• components/libc/posix/io/stdio/stdio.c

🏷️ Tag: components_libc

Reviewers: @GorrayLi @mysterywolf

Changed Files (Click to expand)

• components/libc/posix/io/stdio/stdio.c

---

📊 Current Review Status (Last Updated: 2025-10-11 00:26 CST)

• ⌛ @GorrayLi Pending Review
• ⌛ @Maihuanyi Pending Review
• ⌛ @mysterywolf Pending Review

---

📝 Review Instructions

1. 维护者可以通过单击此处来刷新审查状态: 🔄 刷新状态
   Maintainers can refresh the review status by clicking here: 🔄 Refresh Status

2. 确认审核通过后评论 LGTM/lgtm
   Comment LGTM/lgtm after confirming approval

3. PR合并前需至少一位维护者确认
   PR must be confirmed by at least one maintainer before merging

ℹ️ 刷新CI状态操作需要具备仓库写入权限。
ℹ️ Refresh CI status operation requires repository Write permission.

[BernardXiong]

请使用英文注释

[BernardXiong]

这里的宏判断需要有更清晰的解释，哪个版本有，哪个版本取消。或者说之前的版本方式会更适合些？请给出更多的注释吧，谢谢