KVM: arm64: Generate hyp_constants.h for the host

In order to avoid exposing hypervisor (EL2) data structures directly to
the host, generate hyp_constants.h to provide constants such as structure
sizes to the host without dragging in the definitions themselves.

Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211202171048.26924-3-will@kernel.org