unix: Add squiid platform-dependent code.

Currently, this is mostly copied from redox so it builds for now.
It will be adapted as needed in future.

Signed-off-by: Christoph Heiss <contact@christoph-heiss.at>

src/unix/mod.rs

@@ -1570,6 +1570,9 @@ cfg_if! {
     } else if #[cfg(target_os = "nto")] {
         mod nto;
         pub use self::nto::*;
+    } else if #[cfg(target_os = "squiid")] {
+        mod squiid;
+        pub use self::squiid::*;
     } else {
         // Unknown target_os
     }