[PATCH v3] Revert __HAVE_64B_ATOMICS configure check

[Florian Weimer]

* Adhemerval Zanella Netto:

> On 14/11/25 10:44, Wilco Dijkstra wrote:
>> Hi Adhemerval,
>> 
>>> - Enable USE_64B_ATOMICS for x32.
>> 
>> That's incorrect. There is no guarantee that unaligned accesses are
>> atomic even on x86.
>
> I tend to agree, but this was it was historical done and I prefer to
> *not* change now (and H.J already raised this as well).
>
> I think it would be better to disable a 64-bit atomic on x32 on a
> subsequent patch with a explanation on where it fails (since now
> x32 glibc deployments are using it anyway).

There's always space for a naturally aligned 8-byte quantity in current
sem_t.  I think this is fixable without an ABI bump, it's just ugly.

We can probably assume that external uses of semaphores via sem_init are
fully aligned, so that the alignment step does not alter the
interpretation of frozen sem_t values.

Thanks,
Florian