remove strnlen from rtthread.h
add __STDC_LIMIT_MACROS macro in Keil
add RT_USING_LIBC in simulator
add _DLIB_ADD_EXTRA_SYMBOLS=0 in IAR

strnlen 不属于ISO/ANSI-C标准库的函数
string.h中部分glibc/POSIX拓展函数各个编译平台不一致
例如strnlen 在IAR中定义 但是keil gcc下没有定义
IAR取消扩展C函数声明
取消strnlen函数在rtthread.h中的定义

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• 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
• 本拉取/合并请求代码是高质量的 Code in this PR is of high quality
• 本拉取/合并使用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