qt5: disable qt-5.8.0 support for arc, nios2 and xtensa

qt5: disable qt-5.8.0 support for arc, nios2 and xtensa

Fixes [1], [2], [3]:

  ../3rdparty/double-conversion/include/double-conversion/utils.h:81:2: error: #error Target architecture was not detected as supported by Double-Conversion.

[1] http://autobuild.buildroot.net/results/ce6/ce6f506f610434dff1d0eb64644be0200d7c52f6
[2] http://autobuild.buildroot.net/results/fda/fda8a5165cc599adfa4b3cb012916fa4dc38aeca
[3] http://autobuild.buildroot.net/results/779/779fd562c28f3a47f9caea7d0e0cb1dcda7fe489

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>