`sys.exit` hangs indefinitely

As previously discussed in #174, this is due to https://github.com/WebAssembly/wasi-libc/issues/617, which was fixed in WASI-SDK 28. However, WASI-SDK 28 also broke the `wasm32-wasip2` target in various ways, e.g. https://github.com/WebAssembly/wasi-libc/issues/640, so we'll need to wait for WASI-SDK 29 at least before we can pull that fix in (or backport the fix if we get desperate).