target/arm/helper: zcr: Add build bug next to value range assumption

target/arm/helper: zcr: Add build bug next to value range assumption

The current implementation of ZCR_ELx matches the architecture, only
implementing the lower four bits, with the rest RAZ/WI. This puts
a strict limit on ARM_MAX_VQ of 16. Make sure we don't let ARM_MAX_VQ
grow without a corresponding update here.

Suggested-by: Dave Martin <[email protected]>
Signed-off-by: Andrew Jones <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Eric Auger <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>