libselinux: mark as not available on ARC

libselinux causes some build problems due to the toolchain on ARC,
which haven't been solved so far. As a temporary solution for Buildroot
2015.08, this commit makes libselinux (and its reverse dependencies)
unavailable on ARC. Of course, once the toolchain problem is
addressed, this commit can be reverted to re-enable libselinux on ARC.

Fixes:

   http://autobuild.buildroot.org/results/220/2207f6aad44a6988bf07b02b583b6418ad930dc8/



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