polarssl: disable microblaze inline assembly

Fixes:
http://autobuild.buildroot.net/results/4d5/4d54958ded61a0d929d992e4ca0bb31c996953cb/



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