binutils: do not allow on NIOS2, no support available

binutils does not have support for NIOS2 on the target, so disable the
selection of binutils on this architecture.

Fixes:

  http://autobuild.buildroot.org/results/40d/40d9cbefd03727af82610412ed0bc47674c44dbb/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>