test intrinsic fallback bodies with Miri `@Urgau` noted in https://github.com/rust-lang/rust/pull/140792 that fallback bodies our backends don't use are untested... which is correct, and it is a problem. So this adds a testing-only flag to Miri to force the use of fallback bodies, and adds a run of the Miri test suite with that flag to CI. This should not take much more than a minute so I hope it's fine? Let's see how long it actually takes. While at it, I made that test run also enable MIR optimizations. Miri's CI has a run with that, and it has caught mir-opt bugs in the past -- this way we'd see the CI failure earlier. r? `@scottmcm`