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

src/libcore/lib.rs

@@ -38,7 +38,7 @@
 //!   provided by the [rlibc crate](https://crates.io/crates/rlibc).
 //!
 //! * `rust_begin_unwind` - This function takes three arguments, a
-//!   `fmt::Arguments`, a `&str`, and a `usize`. These three arguments dictate
+//!   `fmt::Arguments`, a `&str`, and a `u32`. These three arguments dictate
 //!   the panic message, the file at which panic was invoked, and the line.
 //!   It is up to consumers of this core library to define this panic
 //!   function; it is only required to never return.