[libc][math] implement `signbit`

[akielaries]

This PR addresses #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.