python-numpy: disable on non-supported architectures

The numpy/core/include/numpy/npy_cpu.h file has some
architecture-specific definitions, and not all architectures are
supported. This commit makes sure python-numpy cannot be selected on
the architectures that are not supported.

Fixes:

  http://autobuild.buildroot.org/results/327/327c85f5b1849d390fcf0727cf48e621b946571f/



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