[CORE] Mandate arch_vcpu_preempt_orphan() for all architectures

This patch finally removes the dependence on timer expire APIs
and make arch_vcpu_preempt_orphan() mandatory for all architectures.

The APIs removed from core code include vmm_timer_event_expire() and
vmm_clockchip_force_expirey().

Signed-off-by: Anup Patel <anup@brainfault.org>