Add comments to the rt-thread\components\libc\compilers\common\cstdlib.c

[shu-qian]

[
The purpose of this PR is to submit the PR to Fixes #7426
and add comments in the source code to improve the readability and understandability of the code and contribute to the project.

I have added detailed comments to rt-thread\components\libc\compilers\common\cstdlib.c

This PR is mainly for comments on the source code and does not involve specific functional changes, so there is no specific testing environment.
]

• 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

[mysterywolf]

此处无需修改

[shu-qian]

Sorry, I've corrected the indentation problem you pointed out.

[mysterywolf]

Thanks for your PR!