package/libev: force arm mode instead of Thumb mode

Fix the following build failure:

/tmp/ccHc3Awn.s:825: Error: selected processor does not support `mcr p15,0,r3,c7,c10,5' in Thumb mode

Fixes:
 - http://autobuild.buildroot.org/results/1c597c9da724d6cac06b09b1ecd456a28440a3a3



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