[kernel]: fix incorrect %hh format output for values in [0x80, 0xff]

[rsj123]

拉取/合并请求描述：(PR description)

为什么提交这份PR (why to submit this PR)

#11171
rt_vsnprintf_tiny 中 %hh 修饰符处理存在 bug，当 int8_t/uint8_t 变量值在 [0x80, 0xff] 区间时，%hhu 输出错误。

exp:

int8_t  i8 = -1;
uint8_t u8 = 255;
rt_kprintf("i8 = %hhu, u8 = %hhu\n", i8, u8);
// 修复前: i8 = 4294967295, u8 = 4294967295
// 修复后: i8 = 255, u8 = 255

你的解决方案是什么 (what is your solution)

原始代码：
在 src/klibc/rt_vsnprintf_tiny.c 第 563 行，qualifier == 'H'（对应 %hh）分支中：

num = (rt_int8_t)va_arg(args, rt_int32_t);

无论有符号还是无符号格式，都先用 (rt_int8_t) 做了有符号截断，导致符号扩展。对于 %hhu 等无符号格式，不会进入 SIGN 分支修正，num 保持错误的 0xFFFFFFFF。

修复：将 (rt_int8_t) 改为 (rt_uint8_t)，先做无符号截断，仅在 SIGN 标志位置位时才做 (rt_int8_t) 符号扩展。

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

• BSP: bsp\qemu-vexpress-a9

• .config: 未改动

• action: https://github.com/rsj123/rt-thread/actions/runs/22237184572

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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

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[github-actions]

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[github-actions]

📌 Code Review Assignment

🏷️ Tag: kernel

Reviewers: @GorrayLi @ReviewSun @hamburger-os @lianux-mm @wdfk-prog @xu18838022837

Changed Files (Click to expand)

• src/klibc/rt_vsnprintf_tiny.c

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📊 Current Review Status (Last Updated: 2026-02-21 03:05 CST)

• ⌛ @GorrayLi Pending Review
• ⌛ @ReviewSun Pending Review
• ⌛ @hamburger-os Pending Review
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