Implement C23 memalignment

[Collin Funk]

Joseph Myers <josmyers@redhat.com> writes:

> +/* Return the alignment of P.  */
> +size_t
> +memalignment (const void *p)
> +{
> +  size_t i = (size_t) p;
> +  return i & -i;
> +}

Doesn't it make more sense to cast the pointer to uintptr_t? In practice
it should be the same as size_t, but theoretically the cast could change
the value and be conformant to ISO C, unlike with uintptr_t.

Patch looks good otherwise, though.

Thanks,
Collin