Fix WASI fd_readdir on big-endian

This code assumes that the Dirent structure has the same memory
layout on the host (Rust code) as in wasm code.  This is not true
if the host is big-endian, as wasm is always little-endian.

Fixed by always byte-swapping Dirent fields to little-endian
before passing them on to wasm code.

crates/wasi-common/src/snapshots/preview_1.rs

@@ -1548,13 +1548,24 @@ fn dirent_bytes(dirent: types::Dirent) -> Vec<u8> {
         std::mem::size_of::<types::Dirent>() as _,
         "Dirent guest repr and host repr should match"
     );
+    assert_eq!(
+        1,
+        std::mem::size_of_val(&dirent.d_type),
+        "Dirent member d_type should be endian-invariant"
+    );
     let size = types::Dirent::guest_size()
         .try_into()
         .expect("Dirent is smaller than 2^32");
     let mut bytes = Vec::with_capacity(size);
     bytes.resize(size, 0);
     let ptr = bytes.as_mut_ptr() as *mut types::Dirent;
-    unsafe { ptr.write_unaligned(dirent) };
+    let guest_dirent = types::Dirent {
+        d_ino: dirent.d_ino.to_le(),
+        d_namlen: dirent.d_namlen.to_le(),
+        d_type: dirent.d_type, // endian-invariant
+        d_next: dirent.d_next.to_le(),
+    };
+    unsafe { ptr.write_unaligned(guest_dirent) };
     bytes
 }