Change realSig to be false by default

[T-Gro]

This changes the default of realSig to be false until #17607 is resolved.
Once a fix for #17607 is in place, it can change the default back to true.

With current state, compiled code could experience performance degradation at runtime.

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[vzarytovskii]

Thanks, it's crucial we ship no known optimiser degradation. It might be too late for rc1 though, but rc2 should be fine.