target-arm: Handle 'smc' as an undefined instruction

Refine check on bkpt so that smc and undefined instruction encodings are
handled as an undefined instruction and trap.

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>
