qt5jsbackend: needs ARM v5+

Just like nodejs and other v8-based javascript engines for ARM interwork
it needs the BLX instruction which is only available on V5+ cores.
Fixes:
http://autobuild.buildroot.net/results/853/853cd29bf427902b2cb82a101964a4e45714cb86/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>