arm64: refine the fatal handler

[lipengfei28]

1. Summary
   The original implement for exception handler is very simple and
   haven't framework for breakpoint/watchpoint routine or brk instruction.
   I refine the fatal handler and add framework for debug handler to
   register or unregister. this is a prepare for watchpoint/breakpoint implement
2. add arm64_current_el to obtain current EL]

Summary

Impact

Testing

[nuttxpr]

[Experimental Bot, please feedback here]

NuttX PR Requirements Review:

Meets Requirements?: Partially

Reasoning:

• Summary: The summary provides a high-level overview but lacks specifics.

  • Needs Improvement: Clearly state the functional part of the code being changed (e.g., exception handling mechanism) and explicitly describe how the change works (e.g., "introduces a registration mechanism for debug handlers, allowing for breakpoint/watchpoint functionality in the future").

• Impact: The "Impact" section is entirely missing.

  • Needs Addition: Address all the bullet points in the "Impact" section of the requirements. Pay particular attention to:
    • New/changed features: Describe the framework for debug handlers.
    • Impact on users: Will users need to modify their code or configuration?
    • Impact on hardware/boards: Are there specific architectures or boards affected?
    • Documentation: Does this change necessitate documentation updates? If so, are they included in the PR?

• Testing: The "Testing" section is incomplete.

  • Needs Improvement:
    • Specify the build host OS, CPU, compiler, and versions used.
    • List the target architectures, boards, and configurations tested.
    • Provide actual testing logs (or links to them) before and after the change.

Recommendations:

1. Expand the Summary: Provide more detailed information about the changes made.
2. Complete the Impact Section: Address each bullet point with specific details relevant to your changes.
3. Fill out the Testing Section: Include concrete information about your testing environment and results.

By addressing these points, you'll make your PR clearer, easier to review, and more likely to be merged.

[pussuw]

LGTM!