toolchain/glibc: add version 2.20

And remove version 2.18 to just keep two versions around.
So bump the default to 2.19.

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