package/x11r7/xserver_xorg-server: libunwind support is broken on mips64

Fixes
http://autobuild.buildroot.net/results/e8f/e8f68163802f31359cee15fa3ca52547e06d1f27/
http://autobuild.buildroot.net/results/994/9947222da8682a358f81f8ac924c12596fd09ee8/
http://autobuild.buildroot.net/results/df9/df92d9b682976b77d9e1e4c39afca52b7512d847/



[Peter: use official bugtracker URL, fix check]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>