glibc: add version 2.21

- Add version 2.21
- Add a hash value for it

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>