cmd: sandbox: implement exception command

Implement the commands

* exception undefined - execute an illegal instruction
* exception sigsegv - cause a segment violation

Here is a possible output:

    => exception undefined
    Illegal instruction
    pc = 0x55eb8d0a7575, pc_reloc = 0x57575
    Resetting ...

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>