Certainly not general CPUs, but there are probably domain specific processors out there that use something else. Why would you want to design a domain specific processor and still use C? Beats me.

Even if you don't really want C on such a process there will be an emergent and unholy aliance between (1) a pointy-hair impulse within the manufucturer to have "programmable in C" on the feature list and (2) an empire-building impulse within the C standards writing ecosystem that wants to encompass every chip under the sun.

Those forces are so strong people are still pushing for C on FPGAs.

The bigger reason it's still undefined is to enable this type of optimization: https://news.ycombinator.com/item?id=14857316

The comment thread seems to suggest that even if you define the behavior you can still optimize that case, and in fact Clang does.