x86: fix boot time APIC mode detection

x86: fix boot time APIC mode detection

current_cpu_data becomes valid only relatively late in the boot
process, so looking there for a particular feature early in the game
would generally give the appearance of the feature being unavailable.

Getting this wrong means that at kexec time the system would get
returned to xAPIC mode, causing disconnect_bsp_APIC() to try to access
the APIC page, which on systems with x2APIC pre-enabled will never get
set up.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>