xen/arm: fix get_cpu_info() when built with clang

xen/arm: fix get_cpu_info() when built with clang

Clang understands the GCCism in use here, but still complains that sp is
unitialised. In such cases, resort to the older versions of this code,
which directly read sp into the temporary variable.

Note that GCCism is still kept in default because other compilers (e.g.
clang) may also define __GNUC__, so AFAIK there are no proper way to
detect properly GCC.

This means that in the event Xen is ported to a new compiler, the code
will need to be updated. But that likely not going to be the only place
where Xen will need to be adapted...

This is based on the x86 counterpart.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>