glibc: disable 2.20 for sparc

Disable glibc 2.20 for sparc, it's broken, that leaves 2.21 as the
default in this case.
Fixes bug #7941.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>