x2APIC: improve enabling logic

x2APIC: improve enabling logic

This patch masks PIC and IOAPIC RTE's before x2APIC enabling, unmask
and restore them after x2APIC enabling. It also really enables
interrupt remapping before x2APIC enabling instead of just checking
interrupt remapping setting. This patch also handles all x2APIC
configuration including BIOS settings and command line
settings. Especially, it handles that BIOS hands over in x2APIC mode
(when there is apic id > 255). It checks if x2APIC is already enabled
by BIOS. If already enabled, it will disable interrupt remapping and
queued invalidation first, then enable them again.

Signed-off-by: Weidong Han <weidong.han@intel.com>