Implement C23 memalignment

[Paul Eggert]

On 2025-10-03 00:49, Andreas Schwab wrote:
> On Okt 03 2025, Paul Eggert wrote:
> 
>> It's still not going to be as efficient as the non-memalignment code,
> 
> Why does it matter?

Yes, the efficiency hit is so small that it typically doesn't matter.

I'm mostly worried about the *understanding* hit. What is memalignment 
*for*? Why is it needed? Why would one use it instead of other obvious 
(and more efficient :-) techniques that have worked since the 1970s? 
It's helpful for the manual to mention this, as it's not at all obvious 
to the typical reader.