Rihanna and her boyfriend A$AP Rocky are reportedly set to release a tribute sing to their tragic friend Fatman Scoop. The ‘Umbrella’ singer, 36, and her 35-year-old rapper partner – with whom she has sons RZA, two, and Riot, 13 months – are apparently two of a string of the late performers’ pals who have got together to make the track after the ‘Lose Control’ singer died aged 53 following his collapse on stage in front of a crowd of shocked fans. A source told The Sun about how Rihanna apparently recorded vocals for the song at the Red Bus Recording Studio in London earlier this week: “Rihann…