package/opus: force arm mode instead of Thumb mode

Fix the following build failure:

/tmp/cckXvlOO.s:352: Error: selected processor does not support `smull r6,ip,r5,r0' in Thumb mode

Fixes:
 - http://autobuild.buildroot.org/results/fdedcc8f68651ec34417d609b40effbac456c80d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: tweak comment]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>