`StdFloat::round` incorrectly states ties round toward zero

[Mukundan314]

The documentation for StdFloat::round indicates that ties (halfway cases) should round toward zero. However, the observed behavior contradicts this; ties are currently rounded away from zero.

Example code to reproduce:

#![feature(portable_simd)]
use std::simd::prelude::*;
use std::simd::StdFloat;

fn main() {
    println!("{:?}", Simd::from_array([1.5]).round());
}

Output:

[2.0]