LoongArch: KVM: Add stub for kvm_arch_vcpu_preempted_in_kernel()

Pause-Loop Exiting is not supported by LoongArch hardware, nor is pv
spinlock feature. So function kvm_vcpu_on_spin() is not used. Function
kvm_arch_vcpu_preempted_in_kernel() is defined as a stub function here
since it is only called by unused function kvm_vcpu_on_spin().

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>