Remove duplicate code

Because relibc is so big it's easy to accidentally add code that already exists. A few examples - but probably not all of them - are:

• pte.rs implements a mutex when we already since two months earlier had a mutex with more documentation
• sys_select/ implements a bit set when we already use a specifically-designed (memory compatible) crate for this

Feel free to comment with more examples. If I see it (even though notifications were broken last time I checked), I'll update this list.