enable mimalloc on wasm

[jingkaimori]

• Before adding new features and new modules, please go to issues to submit the relevant feature description first.
• Write good commit messages and use the same coding conventions as the rest of the project.
• Please commit code to dev branch and we will merge into master branch in feature
• Ensure your edited codes with four spaces instead of TAB.

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• 增加新特性和新模块之前，请先到issues提交相关特性说明，经过讨论评估确认后，再进行相应的代码提交，避免做无用工作。
• 编写友好可读的提交信息，并使用与工程代码相同的代码规范，代码请用4个空格字符代替tab缩进。
• 请提交代码到dev分支，如果通过，我们会在特定时间合并到master分支上。
• 为了规范化提交日志的格式，commit消息，不要用中文，请用英文描述。

[star-hengxing]

Maybe missing wasi.

https://github.com/microsoft/mimalloc/blob/4e50d6714d471b72b2285e25a3df6c92db944593/src/prim/prim.c#L17-L24

[jingkaimori]

__wasi__ is defined at https://github.com/microsoft/mimalloc/blob/4e50d6714d471b72b2285e25a3df6c92db944593/include/mimalloc/internal.h#L673 by detecting __EMSCRIPTEN__ macro

[star-hengxing]

But src/prim/prim.c have not include any headers. So we need to add __wasi__ macro.

[star-hengxing]

microsoft/mimalloc#822
microsoft/mimalloc#812

[star-hengxing]

#5341