`lang_start` in std/src/rt.rs is unsound in presence of panic payload that panics on drop

[nagisa]

See #86027 for an example of the problem.

fn main() {
    std::panic::panic_any(Bomb);
}

thread 'main' panicked at 'Box<Any>', src/main.rs:12:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'explicit panic', src/main.rs:7:9
fatal runtime error: failed to initiate panic, error 5
abort (core dumped)

Here we panic without any landing pads available upstack, because we are already the top-most(-ish) frame in the first place. This is UB.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-critical