gdb: disable on AArch64

The mainline gdb does not yet have AArch64 support at all, so let's
disable it for this platform. The external toolchain provided by
Linaro has gdbserver + cross-gdb, so it already provides what's
necessary to do some debugging.

Fixes
http://autobuild.buildroot.org/results/c330eb75bd2d3e2f002e7a362dd5b08c4fc7fafc/build-end.log

.

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