PowerPC mishandles xscmpudp instruction
Description of problem:
xscmpudp instruction is mishandled
Steps to reproduce:
1. Compile the attached program with VSX (e.g. `RUSTFLAGS=-Ctarget-feature=+vsx cargo build --target=powerpc64-unknown-linux-gnu`)
2. Run the program and expect assertions to pass.
3. See assertions fail.
Additional information:
When VSX is disabled, the `fcmpu` instruction is emitted instead (and handled properly).  See the offending program:
```
pub fn main() {
    use std::hint::black_box;
    assert!(black_box(f64::NAN)
        .clamp(black_box(0f64), black_box(0f64))
        .is_nan());
}
```