support `poll_oneoff`

[gkgoat1]

Go and Rust's tokio require an implementation of poll_oneoff, important parts of their ecosystem.

[turbolent]

So far I have not encountered any WASI WebAssembly programs that use it. Do you maybe have some example Go and Rust programs that use this WASI syscall?

[gkgoat1]

the Go runtime, so any Go program using the official toolchain, such as a "Hello, World".

mio's reactor, so, again, even a simple Tokio "Hello, World".