KVM: arm64: Factor out code for checking (h)VHE mode into a macro

The code for checking whether the kernel is in (h)VHE mode is
repeated, and will be needed again in future patches. Factor it
out in a macro.

No functional change intended.
No change in emitted assembly code intended.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/kvmarm/20230724123829.2929609-3-tabba@google.com/


Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>