torture: Default --no-rcutasksflavors on arm64

torture: Default --no-rcutasksflavors on arm64

Because arm64 does not support CONFIG_SMP=n kernels, --do-rcutasksflavors
gets Kconfig errors when running the TINY01 rcutorture scenario.
This commit therefore makes --no-rcutasksflavors be the default on
arm64.  Once kvm.sh automatically deselects CONFIG_SMP=n rcutorture
scenarios on arm64, the two lines marked "FIXME" can be changed back
from "${ifnotaarch64}" to "yes".

Note that arm64 users can still specify --do-rcutasksflavors in order
to override this default.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>