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

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86518

--- Comment #7 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
cp/mangle.o miscompares due to -Wsign-compare, possibly due to caching in
maybe_constant_value as in the above PR.