Improve conversion of Math functions in user kernels

[wcmjunior]

User kernels are currently being transported to C kernel files without any code analysis. The compiler simply rename variables and replace math functions' names. In this sense, Math.abs function is replaced to "fabs", which is a C version for getting absolute values from float variables. This will cause RenderScript and OpenCL compilation errors in C files once the user deals with Math.abs and integer numbers in Java, since the equivalent function in C is "abs".