Add AsFd implementations for stdio lock types on WASI.

This mirrors the implementations on Unix platforms, and also mirrors the
existing `AsRawFd` impls.

This is similar to rust-lang#100892, but is for the `*Lock` types.

library/std/src/sys/wasi/stdio.rs

@@ -30,6 +30,13 @@ impl AsFd for Stdin {
     }
 }
 
+impl<'a> AsFd for StdinLock<'a> {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(0) }
+    }
+}
+
 impl io::Read for Stdin {
     fn read(&mut self, data: &mut [u8]) -> io::Result<usize> {
         self.read_vectored(&mut [IoSliceMut::new(data)])
@@ -65,6 +72,13 @@ impl AsFd for Stdout {
     }
 }
 
+impl<'a> AsFd for StdoutLock<'a> {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(1) }
+    }
+}
+
 impl io::Write for Stdout {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         self.write_vectored(&[IoSlice::new(data)])
@@ -103,6 +117,13 @@ impl AsFd for Stderr {
     }
 }
 
+impl<'a> AsFd for StderrLock<'a> {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(2) }
+    }
+}
+
 impl io::Write for Stderr {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         self.write_vectored(&[IoSlice::new(data)])