package/cmake: disable on unsupported architectures

Supported architectures are listed at [1].

Fixes:
  http://autobuild.buildroot.net/results/de9/de9021fdc63600536ffa91e4b8718584e3e5702c/
  http://autobuild.buildroot.net/results/0f1/0f1afc62e91ab798fa65685af3dea3caac663f11/

[1] https://github.com/Kitware/CMake/blob/master/Utilities/KWIML/ABI.h.in#L339



[Thomas: rewrap list of supported architectures.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>