fix: Use proper portable copysign polyfill

[MaxGraey]

Previous one can't properly handle such edge cases:

copysign(1.0, +0.0)   ->  +0.0
copysign(1.0, -0.0)   ->  -0.0
copysign(1.0, +NaN)   ->  NaN
copysign(1.0, -NaN)   ->  NaN

Now, these cases don't change destination x argument. Only its sign:

copysign(1.0, +0.0)   ->  +1
copysign(1.0, -0.0)   ->  -1
copysign(1.0, +NaN)   ->  +1
copysign(1.0, -NaN)   ->  -1

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