[CPU] add fused_gdn_gating kernel for Qwen3-next

[blzheng]

Motivation

This pr adds fused_gdn_gating kernel for Qwen3-next on CPU.

Test Plan:
python test/srt/cpu/test_mamba.py -k test_fused_gdn_gating

Modifications

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.

[blzheng]

Closing — included in #12324.