KVM: arm64: Add build-time sanity checks for flags

Flags are great, but flags can also be dangerous: it is easy
to encode a flag that is bigger than its container (unless the
container is a u64), and it is easy to construct a flag value
that doesn't fit in the mask that is associated with it.

Add a couple of build-time sanity checks that ensure we catch
these two cases.

Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>