threads.h implementation issues

[Roland McGrath]

> 3. Cast int->uintptr_t->void*->uintptr_t->int to round-trip the int
>    result via a value of type void*. This assumes that any value of
>    int can be represented in uintptr_t, and that any value of
>    uintptr_t can be represented in void*. I think this is acceptable
>    but others may disagree.

That is a valid assumption we make all over the place.  
This is the right thing to do.