kernel.spec: xz compress modules on i686 and x86_64
    
    Pretty grody, but it seems to be working... it has to happen after
    module signing (obviously) and after find-debuginfo.sh runs as well, so
    tacking it onto the end seems sensible, and then just fixing up the file
    lists as we go. Provides a nice tidy savings to the disk footprint:
    
    kyle@dreadnought:~% for i in kernel-core-3.15.0-0.rc3.git4.1.fc21.x86_64.rpm \
    kernel-core-3.15.0-0.rc3.git4.1.fc21.x86_64.rpm.1; do \
     rpm -qip $i | grep '^Size'; done
    Size        : 43011603
    Size        : 81106737
    
    kmod has handed .xz and .gz modules for a long time now, and can cope.
    
    I was thinking we might also %ghost the .ko, and have a %post install
    script that un-xz the modules in /proc/modules, but none of them are big
    enough for that to be worthwhile I think (maybe XFS, but I doubt it.)
    
    --Kyle