Support "readdir" on Linux (and all Unixes)

[RalfJung]

We currently support readdir64 on Linux but readdir only on other Unixes where we need it for the standard library. We should support "readdir" everywhere ("readdir64" is Linux-specific, so that remains only there).

This is similar to #4684 so the PR will likely look a lot like #4714:

• add a new case in unix/foreign_item.rs and remove the now-redundant cases in the other files
• add a test directly invoking readdir to ensure it actually works