ObjFW  Check-in [6b15bb229e]

* of_atomic_cmpswap_*: Clobber eax by adding it as an (unused) output,
so the compiler resets eax (e.g. when used in a loop). (Adding it
to the input and clobber list is lot legal!)
* Don't dereference the pointer passed using "m"(*p) twice.
* Fix some code for ILP64 systems (it seems there is no architecture
using it, though, as it even contained syntax errors).
* Use __asm__ __volatile__ just to be sure.

Thanks to Florian Zeitz for pointing out most of these issues!