[PATCH 21/22] Define __HAVE_64B_ATOMICS from compiler support

[Wilco Dijkstra]

Hi Adhemerval,
 
> Now that atomic builtins are used by default, we can rely on the
> compiler to define when to use 64-bit atomic operations.
>
> It allows the use of 64-bit atomic operations on some 32-bit ABIs where
> they were not previously enabled due to missing pre-processor handling:
> hppa, mips64n32, s390, and sparcv9.
>
> Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

LGTM.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Cheers,
Wilco