sandbox: make function 'do_undefined' properly compiles for PowerPC

The 2 bytes 0xffff is too short for being a PowerPC instruction, resulting
in an error similar to:
    /tmp/ccW8yjie.s: Assembler messages:
    /tmp/ccW8yjie.s: Error: unaligned opcodes detected in executable segment
    /tmp/ccW8yjie.s:223: Error: instruction address is not a multiple of 4
    make[2]: *** [/tmp/ccyF4HIC.mk:17: /tmp/ccCKUFuF.ltrans5.ltrans.o] Error 1

Signed-off-by: WHR <msl0000023508@gmail.com>