libev: also fails for blackfin

Also fails for blackfin because of a lack of memory fences so disable
it. Fixes:
http://autobuild.buildroot.net/results/d895f0b0a5f4b6c173c2843a76277e6fe611627b/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>