[Bug target/66120] __builtin_add/sub_overflow for int32_t emit poor code on ARM

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66120

--- Comment #4 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
Mul doesn't produce useful overflow bits when the flags are set.  We could do
negv3.