vmm: arch: Introduce RISC-V regs

[TimePrinciple]

Changes

Implement macros and methods used to g/set RISC-V registers.

Reason

Introduce RISC-V architecture support.

License Acceptance

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PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

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• This functionality cannot be added in rust-vmm.