vo-aacenc: pass -marm to build non-Thumb2 compatible assembly

vo-aacenc has some hand-written assembly that cannot build in Thumb2,
so we have to explicitly pass -marm to make sure this particular
package is always built with full-length ARM instructions.

Fixes:

  http://autobuild.buildroot.org/results/680/680b29cd824624eb8e4ec71187b9a6576444e72b/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>