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 <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>