Rollup merge of #129683 - RalfJung:copysign, r=thomcc

copysign with sign being a NaN can have non-portable results

Follow-up to rust-lang/rust#129559.
Cc ```@tgross35``` ```@beetrees```

There's no portable variant we can recommend instead here, is there? Something with a semantics like "if `sign` is a NaN, then return `self` unaltered, otherwise return `self` with the sign changed to that of `sign`"?