Update to latest WASI-SDK and the new `wasm32-wasip2` target

[dicej]

Now that Rust supports the wasm32-wasip2 target, we should be able to stop using my fork of WASI-SDK and instead use the latest upstream release. Also, IIRC, Rust builds with -fPIC by default for wasm32-wasip2, so we might be able to stop using the -Z build-std flag to rustc and drop the dependency on nightly.