All languages have strengths and weaknesses - bit manipulation is a weakness in 'C'.

There is no problem interfacing Keil C51 and assembler (there's a whole chapter on it in the Manual) - so just use the language that is most appropriate to the task in hand.

There is no point making life difficult for yourself just for the sake of sticking to 'C'.

I would probably write a small assembler function to do this.