[libcpu][cortex-a] Refactor FPU stack initialization for maintainability

[Copilot]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

Referenced #10906 which fixed FPU stack initialization bugs. This PR refactors the implementation for better code quality while maintaining identical functionality.

你的解决方案是什么 (what is your solution)

Refactored FPU initialization using constants and rt_memset:

1. Added VFP_DATA_NR constant (64 words) in cpuport.h following cortex-r4 pattern
2. Replaced hardcoded initialization with rt_memset for cleaner code
3. Improved comments to clarify stack layout matching context_gcc.S

Before:

#ifdef RT_USING_FPU
    *(--stk) = 0;       /* not use fpu*/
#endif

After:

#ifdef RT_USING_FPU
    /* FPU context initialization matches context_gcc.S restore order:
     * Stack layout (high to low): FPEXC -> FPSCR -> D16-D31 -> D0-D15
     */
    stk -= VFP_DATA_NR;
    rt_memset(stk, 0, VFP_DATA_NR * sizeof(rt_uint32_t));  /* Initialize D0-D31 (64 words for 32 double regs) */
    *(--stk) = 0;       /* FPSCR: Floating-Point Status and Control Register */
    *(--stk) = 0x40000000;  /* FPEXC: Enable FPU (bit 30 = EN) */
#endif

Stack layout (high to low): FPEXC (1 word) → FPSCR (1 word) → D16-D31 (32 words) → D0-D15 (32 words)

Changes:

• libcpu/arm/cortex-a/cpuport.h: +5 lines (VFP_DATA_NR definition)
• libcpu/arm/cortex-a/stack.c: Uses rt_memset to initialize FPU registers, no loop variable needed

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

• BSP: Code-only refactoring, no functional change from [libcpu][arm] fix: correct FPU stack initialization order and alignment on Cortex-A #10906

• .config:

  • CONFIG_RT_USING_FPU=y (Cortex-A only)

• action: N/A (architecture-specific, requires Cortex-A hardware/QEMU)

]

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

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug] Cortex-A中线程栈FPU情况下的初始化问题</issue_title>
<issue_description>### RT-Thread Version

master

Hardware Type/Architectures

cortex-a

Develop Toolchain

GCC

Describe the bug

参加 #10906

需要有更好的代码实现方式处理好Cortex-A中线程栈FPU下初始化的问题

Other additional context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug] Cortex-A中线程栈FPU情况下的初始化问题 #10918

---

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[BernardXiong]

@copilot 不希望定义 i 变量，可以直接使用 rt_memset 的方式取代循环。

[Copilot]

已修改为使用 rt_memset 替代循环，并移除了 i 变量定义。修改已提交至 06eae47。

Changed to use rt_memset instead of loop and removed the i variable definition. Committed in 06eae47.

[BernardXiong]

cc @Xenithya

[Xenithya]

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