rcutorture: Fix issue with re-using old images on ARM64

On ARM64, when running with --configs '36*SRCU-P', I noticed that only 1 instance
instead of 36 for starting.

Fix it by checking for Image files, instead of bzImage which ARM does
not seem to have. With this I see all 36 instances running at the same
time in the batch.

Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>