fix cfg(target_word_size="32") build

    $ rustc --version
    rustc 0.11-pre-nightly (d35804e 2014-04-18 00:01:22 -0700)
    host: x86_64-unknown-linux-gnu
    $ rustc --target i386-intel-linux --cfg libc core/lib.rs --out-dir . -O -Z no-landing-pads
    core/uint.rs:55:18: 55:26 error: mismatched types: expected `u32` but found `i32` (expected u32 but found i32)
    core/uint.rs:55     ::i32::bswap(x as i32) as uint
                                     ^~~~~~~~
    core/int.rs:55:18: 55:26 error: mismatched types: expected `u32` but found `i32` (expected u32 but found i32)
    core/int.rs:55     ::i32::bswap(x as i32) as int
                                    ^~~~~~~~
    error: aborting due to 2 previous errors