gdb: target gdb not available on microblaze

Fixes

  http://autobuild.buildroot.org/results/5950c95ca12721dd910ad72fe52bfa9ac45a5120/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>