You can verify that your CPU is multiply correctly at one moment in time.

Yes, but active attacks where a third-party can switch on-and-off features on your CPU are obviously vastly different from a passive attack such as a compromised random number source.

The RNG attack suggested can pass verifiability while at the exact same time introduce a backdoor and thus constantly be on.

Are you ready to test all the 2^128 possible multiplications your CPU can do?

Integers form a closed group (in the group theory sense) under multiplication and addition, so if you were concerned about specific calculations being compromised you could verify the answer via alternative calculations and testing for consistency.

Faking consistency is likely impossible without causing a huge amount other calculations (which the CPU will do as part of day-to-day operations) to fail.

Not an expert, but in theory the CPU can detect specific contexts and change its behavior. If side-channel attacks can detect AES operations, the CPU can do an even a better job.