msgpack: disable for ARC

msgpack uses the __sync_fetch_and_add() atomic primitive that is not
available for ARC. Fixes:
http://autobuild.buildroot.net/results/829/8292e67354602738cae12d8ebea83b513ba35482/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>