[PATCH v2] handle sem_t with ILP32 and __HAVE_64B_ATOMICS

[Roland McGrath]

> Is __HAVE_64B_ATOMICS used *only* for semaphores?  Because if there are 
> other effects of changing it, doing so seems risky - it would be better to 
> have a define such as SEM_USE_64B_ATOMICS, and make that default to 0 for 
> ILP32 architectures.

A trivial grep shows there are no other uses.