qt: fix build on powerpc_e500mc

gcc bug internal compiler error: in validate_condition_mode, at
config/rs6000/rs6000.c:180744. Bug is fixed since gcc 7.
Workaround is to set -mno-isel, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60818 and
https://gcc.gnu.org/ml/gcc-patches/2016-02/msg01036.html

This bug is also raised on BR2_powerpc_8540, BR2_powerpc_8548 and
BR2_powerpc_e5500.

Fixes:
 - http://autobuild.buildroot.net/results/9b9d11b3281a72c8f54fc675408acb96d24d8e7e



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>