Customizable `__stack_chk_fail` implementation

[Freddie Chopin]

On Tue, 2019-10-15 at 10:25 +0000, Łukasz Żak wrote:
> When however I build and run the same sources
> for real hardware the problems are detected - I can observe the calls
> to `__stack_chk_fail` with the gdb but as there is no semihosting
> available the write/_exit procedures used by the `__stack_chk_fail`
> handler are basically noops as they should be as on real hardware
> there is no standard output (at least in my case) and there is
> nothing to exit to.

While this is true for _write(), it's wrong for _exit(), which should
_NOT_ return.

This is how a stub for _exit() should look like (code from my C++ RTOS
for ARM microcontrollers):
https://github.com/DISTORTEC/distortos/blob/6c1e232f777284f6ecb935b7f2e94faba6174860/source/newlib/syscallsStubs.cpp#L57

It may reset your chip, it may print/log whatever you want, send you an
e-mail or print something on LCD, but it should never ever return.

Regards,
FCh