KVM: selftests: Generate sysreg-defs.h and add to include path

Start generating sysreg-defs.h for arm64 builds in anticipation of
updating sysreg.h to a version that depends on it.

Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231011195740.3349631-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>