[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 #4 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Yep, that's correct: -Wno-narrowing is necessary for build to succeed at all.