Fixed the behaviour of including math.h/cmath in C++

[ZERICO2005]

Changes:

• Functions like isnan and islessequal now use _Generic in C, and use overloads in C++. Additionally, the return value is casted to int in C, and bool in C++
• All math.h functions can be used in the std:: namespace
• <cmath> no longer includes <type_traits>, instead using <__cmath_type_traits>. __cmath_is_integral being identical to is_integral
• Including <math.h> in C++ now includes <cmath>