target-arm: Decode SETEND correctly in Thumb

target-arm: Decode SETEND correctly in Thumb

Decode the SETEND instruction correctly in Thumb mode,
rather than accidentally treating it like CPS. We don't
support BE8 mode, but this change brings the Thumb mode
in to line with behaviour in ARM mode: 'SETEND BE' is
not supported and will provoke an UNDEF exception, but
'SETEND LE' is correctly handled as a no-op.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Daniel Forsgren <daniel.forsgren@enea.com>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>