gdb: not available on NIOS II

The mainline gdb does not have support for NIOS II, even just for
gdbserver.

Fixes:

  http://autobuild.buildroot.org/results/e57/e57aa016d6a73e59c404a7eb43c7bf5c16cbcbdc/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>