[Bug bootstrap/86518] Strengthen bootstrap comparison by not enabling warnings at stage3

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86518
Bug 86518 depends on bug 86567, which changed state.

Bug 86567 Summary: [8/9 Regression] -Wnonnull/-Wformat/-Wrestrict affect code generation
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86567

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED