Empty default exception handler

[robamu]

The current default handlers use semihosting.
What do you think about making them as simple as possible by just doing a nop loop?
Semihosting support might not work for all use-cases. For example, I am not fully sure whether I can make it
work for my Zynq7000 based board.

[jonathanpallant]

That's a fair point - semihosting isn't a good default for everyone. They should be empty by default, but we should have examples of how to write better ones.

[jonathanpallant]

I think we also want a default Rust handler for all exceptions, rather than just asm ones. I think we discussed this somewhere ... Matrix?