target-s390x: support non current ASC in s390_cpu_handle_mmu_fault

target-s390x: support non current ASC in s390_cpu_handle_mmu_fault

s390_cpu_handle_mmu_fault currently looks at the current ASC mode
defined in PSW mask instead of the MMU index. This prevent emulating
easily instructions using a specific ASC mode. Fix that by using the
MMU index converted back to ASC using the just added cpu_mmu_idx_to_asc
function.

Signed-off-by: Aurelien Jarno <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>