Auto merge of #24323 - rprichard:panic-line-type, r=alexcrichton

There are syntax extensions that call `std::rt::begin_unwind` passing it a `usize`.  I updated the syntax extension to instead pass `u32`, but for bootstrapping reasons, I needed to create a `#[cfg(stage0)]` version of `std::rt::begin_unwind` and therefore also `panic!`.