KVM: arm64: Prepare to handle deferred save/restore of 32-bit registers

32-bit registers are not used by a 64-bit host kernel and can be
deferred, but we need to rework the accesses to these register to access
the latest values depending on whether or not guest system registers are
loaded on the CPU or only reside in memory.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>