Change the rt::unwind line argument type from usize to u32.

[rprichard]

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!.

[rust-highfive]

r? @aturon

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[rprichard]

I suppose I should provide a little more context. The type of line! and column! was changed from usize to u32 in PR #21769. The type of the line value has already been updated in the core::panicking functions, but not in the std::rt::unwind functions.

[alexcrichton]

@bors: r+ ef25b7d

Thanks!

[alexcrichton]

@bors: r+ ef25b7d

[bors]

⌛ Testing commit ef25b7d with merge 49798c5...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt