torture: Run Linux-kernel binary out of results directory

The rcutorture scripts create a qemu-cmd script containing the actual
qemu command.  However, this command references the build directory,
which will be overwritten by later builds.  This commit therefore runs
the kernel out of the results directory so that less hand-editing is
required to re-run a previous test.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>