[mono][debugger] Fix assertion when stepping

[thaystg]

Using the same logic used on amd64.
Fixes #110016

[Copilot]

Pull Request Overview

This PR fixes assertion failures when stepping in the Mono debugger on ARM64 by aligning the breakpoint address calculation logic with the AMD64 implementation. The changes remove the division by 4 that was causing incorrect indexing into the breakpoint addresses array.

• Removes division by 4 from the breakpoint trampoline offset calculation
• Updates array indexing to use native_offset directly instead of native_offset/4
• Eliminates assertions that required 4-byte alignment

[dotnet-policy-service]

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[thaystg]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/16783124435

[steveisok]

LGTM

[thaystg]

/ba-g unrelated failure

[BrzVlad]

@thaystg I don't really understand why the code offset would ever not be 4 byte aligned. I thought all arm64 instructions are 4 bytes ?