nettle: disable assembly optimizations for ARMv7M

It's thumb2-only and it requires ARM instructions.
Since V4 and V5 aren't enough either use the V7M knob to avoid
over-complicating the conditional. Fixes:
http://autobuild.buildroot.net/results/354/35418d33efa902d3a1a82b2cd58d8db1b1172e49/



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