x86: properly define size_t

x86: properly define size_t

Having it defined unilaterally as 'unsigned long' got me surprised
recently when I tried to use the 'z' printk type modifier, as that is
expected by the compiler to be used only on the type it knows size_t
is supposed to have.

Generally the compiler provides a construct to do this, so use it when
available.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Christoph Egger <Christoph.Egger@amd.com>