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?

檢視 issue #128628 和現有的 emscripten 函式指標轉型方法，然後研究如何為 WASI 目標產生 wasm-gc 指令。該 issue 指出 Clang C 前端和後端缺少支援；當指標轉型能在包含 wasm-gc 的 WASI 目標上運作時，即表示完成。