gcc: switch microblaze to version 4.9.x

Switch to gcc 4.9.x for microblaze since it's a better target than
4.8.x, and also add a build patch that fixes (e)glibc build issues.
Hence disable 4.8.x for microblaze.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>