`sign` kernel colide with the OpenCL `sign` function

[StRigaud]

Generating tests for the sign kernel, i faced an overload error between clesperanto sign.cl and the OpenCL sign built-in function.

We would ideally like to rely on the built-in function but:

• it replaces NaN with 0.0
• CUDA function correspondent is name differently: signbit()
• CUDA function return True/False output

Open to rename it differently than pixel_sign in future if better options are proposed.