x86/apic: Add helpers to query current APIC state, e.g. xAPIC vs. x2APIC

Add helpers to query APIC state and use them to replace a bunch of open
coded instances.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221001011301.2077437-4-seanjc@google.com