Add XNNPACK, MobileNetV2, MobileBERT, Llama, ResNet18 to RISC-V testing matrix

[luhenry]

Summary

This is the continuation of #19399 and #19521, to deliver on Phase 2 of #18991

Test plan

This code is exclusively test code. Everything works out of the box, and CI will validate.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19617

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⚠️ 18 Awaiting Approval

As of commit 4b5e42e with merge base 7355d7b ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build Aarch64 Linux Wheels (gh)
• Build documentation (gh)
• Build Linux Wheels (gh)
• Build macOS Wheels (gh)
• Build Presets (gh)
• Build Windows Wheels (gh)
• Cadence Build & Test (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• RISC-V (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test XNNPACK Backend (gh)
• trunk (gh)
• Windows MSVC Build (gh)

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[luhenry]

cc @rascani @mergennachin, and @GregoryComer @digantdesai for XNNPACK

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