target-lm32: kill cpu_abort() calls

Instead of killing QEMU, translate instructions which are not available on
the CPU model as a noop and issue a log message at translation time.

On the real hardware CPU unknown opcodes results in undefined behaviour.

These changes prepare the removal of CPULM32State from DisasContext.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andreas Färber <afaerber@suse.de>
