add panic-probe

[japaric]

this panic handler makes probe-run exit with exit-code=134 (SIGABRT)
closes #41

$ cd nrf52
$ cargo rb panic
0.000000 INFO main
stack backtrace:
   0: 0x000002be - __bkpt
   1: 0x000002bc - panic_probe::__cortex_m_rt_HardFault
   2: 0x0000036a - HardFault
      <exception entry>
   3: 0x000002d0 - __udf
   4: 0x000002b2 - cortex_m::asm::udf
   5: 0x000002a8 - rust_begin_unwind
   6: 0x0000025a - core::panicking::panic_fmt
   7: 0x00000250 - core::panicking::panic
   8: 0x00000162 - panic::__cortex_m_rt_main
   9: 0x00000108 - main
  10: 0x00000286 - Reset
$ echo $?
134

[jonas-schievink]

Nice! Would be good to test this behavior though.

[japaric]

"Why 134/SIGABRT?" -> no particular reason. I see that std::panic! yields a 101 exit code so we might as well use that value.