package/libmad: force arm mode instead of Thumb mode

Fix the following build failure:

/tmp/ccv8mDeW.s:93: Error: selected processor does not support `smull r6,r7,r3,r1' in Thumb mode

Fixes:
 - http://autobuild.buildroot.org/results/92a3f19b4fa12a7724f9e3d99284da4ad2beecb2



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>