Use wasm-gc to handle pointer casts in wasi

In #128628 I switched the way of handling function pointer casts in emscripten to an approach based on wasm-gc. Unlike the previous approaches this should work on the wasi targets, at least the ones that include wasm-gc. There's some difficulty in that there's no support for the instructions in the clang C front end, nor really in the backend yet. So I'm not sure what is the best way to emit these instructions. We only would need to generate one short function using them though.

@brettcannon WDYT?

查看 issue #128628 和现有的 emscripten 函数指针转换方案，然后研究如何为 WASI 目标生成 wasm-gc 指令。该 issue 指出 Clang C 前端和后端缺少支持；当指针转换能够在包含 wasm-gc 的 WASI 目标上正常工作时，即视为完成。