cargo: compile C for wasm32-wasip2 with a WASI clang

The Nix package, the dev shell, and the test check set CC_wasm32_wasip2
and AR_wasm32_wasip2 to the clang of pkgsCross.wasi32 (wasi-libc), so
build scripts with the cc crate work, for example rusqlite with bundled
SQLite. The runtime-toolchain check links C into a probe with the
packaged host and an empty PATH.

cli/tests/crates/cargo-c/src/main.rs

@@ -0,0 +1,25 @@
+//! Adds two numbers with a C function: `{"type": "add", "a", "b"}` -> `{"type": "sum", "value"}`.
+use sausage_actor::{json, messages, reply};
+
+unsafe extern "C" {
+    fn c_add(a: i32, b: i32) -> i32;
+}
+
+fn add(a: i32, b: i32) -> i32 {
+    unsafe { c_add(a, b) }
+}
+
+fn main() {
+    for message in messages() {
+        let (a, b) = (message.body["a"].as_i64().unwrap_or(0), message.body["b"].as_i64().unwrap_or(0));
+        let _ = reply(&message, json!({"type": "sum", "value": add(a as i32, b as i32)}));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn c_code_runs_on_the_host_target() {
+        assert_eq!(super::add(2, 3), 5);
+    }
+}