rcutorture: Avoid torture.sh compressing identical files

Currently, torture.sh will compress the vmlinux files for KASAN and
KCSAN runs.  But it will compress all of the files, including those
copied verbatim by the kvm-again.sh script.  Compression takes around ten
minutes, so this is not a good thing.  This commit therefore compresses
only one of a given set of identical vmlinux files, and then hard-links
it to the directories produced by kvm-again.sh.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>