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

[redi at gcc dot gnu.org]

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

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Eric Gallager from comment #10)
> If this is becoming the meta-bug for all warnings that affect codegen, then
> I'd like to add bug 61579 (-Wwrite-strings) as at least semi-related... (not
> sure if toggling it causes miscompares though)

That one is *supposed* to affect codegen (so maybe it should be a -f option
instead).