gdb: not available on aarch64 for the target

Fixes

  http://autobuild.buildroot.org/results/d32a0fb1584a125cccde521a3960fb87ff7e2de6/build-end.log



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