[PATCH] support: add __address_diff function

[Florian Weimer]

* Yury Khrustalev:

>> The #ifdef is not necessary, sysdeps overrides should work for support/
>> headers.
>
> I've tried this but it doesn't seem to be working. Ideally, I want to
> have the support_pointer_arith.c file only under sysdeps/aarch64 because
> for all other targets the PTR_DIFF macro should be used, but makefile in
> the support folder should probably be modified for this?

I don't know what exactly you tried to do.  I recall that it worked for
me in the past, but I don't remember the details.  We only use sysdeps
overrides for compilation flags today AFAICS.

Thanks,
Florian