build: support armv81 arch in Makefile

[ztechenbo]

build: support armv81 arch detection in Makefile

📝 Description

This PR updates the Makefile Linux architecture detection to recognize armv81 and map it to arm64, so builds work correctly on systems reporting uname -m = armv81.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

N/A

📚 Technical Context (Skip for Docs)

• Reference URL: N/A
• Reasoning: Some environments report uname -m as armv81. Without this mapping, the build target arch may be mis-detected, leading to incorrect output naming/packaging. This change treats armv81 as arm64.

🧪 Test Environment

• Hardware: N/A
• OS: Linux
• Model/Provider: N/A
• Channels: N/A

📸 Evidence (Optional)

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Built successfully after updating arch detection for armv81.

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[nikolasdehor]

Straightforward 2-line Makefile change to map armv81 to arm64. Follows the existing pattern for aarch64. LGTM.

One thought: there are also armv8l variants (lowercase L) reported by some kernels (e.g., 32-bit userspace on 64-bit ARM). If the goal is broad ARMv8 coverage, that might be worth adding too, but that can be a separate PR.