x86: enable interrupts explicitly in __start_xen()

x86: enable interrupts explicitly in __start_xen()

Instead of relying on smp_prepare_cpus() (via check_nmi_watchdog()) or
init_xen_time() (via init_platform_timer() -> plt_overflow())
implicitly enabling interrupts, enable them explicitly once safe to do
so (it may actually be possible to move this even further up, but I
don't think that would buy us much).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Also move spin_debug_enable() a bit higer. Moving it above
smp_prepare_cpus() didn't work for some reason though!

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>