[crashing] Improve crash chaining (#20018)
With this change, `mono_post_native_crash_handler` chains the crashing signal directly, instead of that having to happen after `mono_handle_native_crash` returns (which created a fragile repetitive code pattern). Finally, this way `mono_post_native_crash_handler` can also re-raise the signal right after chaining it. Since we remove our crashing signal handlers in `mono_handle_native_crash`, this should allow the OS to handle the signal as the final action.
Fixes https://github.com/mono/mono/issues/19860