configs: Disable MGSC for SC benefit ablation

[jensen-yan]

Summary

• Disable the whole MGSC predictor in the KMHV3 DecoupledBPUWithBTB config.
• Keep individual SC table settings untouched; this PR only measures whole-SC contribution on current xs-dev.

Motivation

PR #852 previously showed about a 0.3 Int-score drop when SC was effectively removed. Since current mainline includes the recent PHR recovery fix, rerun the simplest SC-off ablation first before deciding whether table-level ablations are worth repeating.

Validation

• python3 -m py_compile configs/example/kmhv3.py
• git diff --check

Expected follow-up: compare the perf robot score against current xs-dev.

Perf run: https://github.com/OpenXiangShan/GEM5/actions/runs/26746954461

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[github-actions]

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (xs-dev)	2.2906	-
This PR	2.2906	➡️ 0.0000 (0.00%)

✅ Difftest smoke test passed!

[XiangShanRobot]

[Generated by GEM5 Performance Robot]
commit: 45cc5aa
workflow: gem5 Align BTB Performance Test(0.3c)

Align BTB Performance

Overall Score

	PR	Master	Diff(%)
Score	18.65	18.90	-1.36 🔴