armv8m: make the debugger handle better faults through ICSR

[mean00]

Detailed description

The ArmV8m does not handle faults the same way. The two main differences are :

• The fault will trigger BEFORE executing the fault rather than after
• The info is split between between DFSR and the new ICSR

As a result the current code does not handle fault very well.
Put a breakpoint on a fault (for example asm("udf #0") and try to "si" on it.

Gdb will appear to get stuck there because the debugger will see the state BEFORE the fault is actually executed and as a result not detect the fault.

The MR adds a separate path that , for ArmV8M, it checks if a fault is pending on ICSR
The "si" command on a fault now works and you proceed on to the handler.

I tested it on a RP2350 and on another CortexM33 successfully.
I'm not convinced all cases are covered as some parts are configurable, but at least it's working better.

Not sure either if the new two new registers should be renamed with V8M in the name or something.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

N/A

[dragonmux]

Got a few notes on going through this PR - with them fixed though we'll be happy to merge this.

[mean00]

I'll update the MR this weekend
Thank you

[mean00]

I think i got all the remarks as a separate commit for easy review
If that's ok, i'll squash them, it takes 10 sec to do so
Thank you

[dragonmux]

This is looking better - there is one small note we have and once that's been addressed, we'll approve and merge this.

[mean00]

This time i think it's correct. I've squashed it as the last change was really small & easy to check.
(slightly altered the commit comment to be in line with the naming for ARMv8-M)
Thank you.

[dragonmux]

It would technically be cortexm: for the commit message suffix as it's not based on the concept touched, but which file is touched.. however, we're going to approve and merge this as it stands as it looks good.

[dragonmux]

LGTM. Thank you for the contribution!

We'll do some cleanup after in a follow-up PR we have to do anyway (comment style, if with assign to true into straight truthy assignment) but this looks all functional which is the important part.