This PR resolves#96322 and implements the `signbit` macro under a new
header `generic-math-macros.h`. This also removed the `TODO` in
`math-macros.h` and moves `isfinite`, `isinf`, and `isnan` to the same
generic maths header. Finally, a test file
`generic-math-macros_test.cpp` that adds coverage to the above 4 macros.
Fixes#96322.