@winni,
I found it in math.pas, doing ctrl+click in the code I jump there even if ti seems not reachable during debug pressing F7, but it seems active like if FPC_MATH_HAS_ARCTAN2 were not defined ($ifndef...).
I was expecting something like you showed, where is located such function ?
here laz 2.0.10, fpc 3.2.0, win 10.