mention rust_panic in default panic! message

[pnkfelix]

Rust developers are developers. Many will be used to using a debugger. Lets tell them straight up from the panic! message how they can hook up their debugger without having to know anything about rust's name mangling algorithm.

[ranma42]

Used as workaround in #30625, but still discovered from other people instead of documentation.
#30625. Mentioning rust_panic in the panic message and possibly in the documentation for panic! might be a good idea. If nobody is already doing this, I will :)