target/arm: Handle SVE vector length changes in system mode

target/arm: Handle SVE vector length changes in system mode

SVE vector length can change when changing EL, or when writing
to one of the ZCR_ELn registers.

For correctness, our implementation requires that predicate bits
that are inaccessible are never set.  Which means noticing length
changes and zeroing the appropriate register bits.

Tested-by: Laurent Desnogues <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 20181005175350[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>