Rollup merge of #105998 - RalfJung:no-unwind-panic-msg, r=thomcc

adjust message on non-unwinding panic

"thread panicked while panicking" is just plain wrong in case this is a non-unwinding panic, such as
- a panic out of a `nounwind` function
- the sanity checks we have in `mem::uninitialized` and `mem::zeroed`
- the optional debug assertion in various unsafe std library functions