largs floats print slightly wrong

[ben0x539]

Lowest roundtripping-integral positive examples I found:

extern {
    fn printf(fmt: *const u8, ...);
}

fn main() {
    let f1: f32 = 41943048_u32 as f32;
    let f2: f64 = 22517998136852488_u64 as f64;
    println!("rust: {}, {}", f1, f2);
    unsafe { printf("c:    %.0f, %.0f\n\0".as_ptr(), f1 as f64, f2); }
}

prints

rust: 41943058, 22517998136852498
c:    41943048, 22517998136852488

We're well past where integers can be roundtripped through floats precisely, but these two examples in particular do roundtrip, so it's weird that we're off by ten when printing them, and anyway C gets it right.

cc #6220?

[emberian]

cc @lifthrasiir the new "strconv" stuff will fix this.

[lifthrasiir]

Now fixed by #24612.

[bstrie]

Closed by #24612.