Correctly unpack &mut [T] into *mut T and usize

wasi-common-cbindgen/tests/val_args.rs

@@ -9,4 +9,4 @@ fn val_args(a: usize, b: usize) -> usize {
 
 fn main() {
     assert_eq!(unsafe { __wasi_val_args(1, 2) }, val_args(1, 2));
-}
+}