Run targeted MLAS tests across CPU architectures (#33054)

### Description

- **CTest integration**
- Add opt-in registration for a focused MLAS FP16 and activation test
subset.
- **CI coverage**
  - Enable the subset once for x86, x64, and ARM64 PR CI.
  - Keep the full, long-running MLAS suite out of per-PR workflows.

### Motivation and Context

CI builds `onnxruntime_mlas_test` but never executes it, leaving MLAS
regressions undetected. This adds representative cross-architecture
coverage while keeping CI runtime reasonable.

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- Fixes #33052

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tianleiwu <30328909+tianleiwu@users.noreply.github.com>