scftorture: Add RPC-like IPI tests

This commit adds the single_weight_rpc module parameter, which causes the
IPI handler to awaken the IPI sender.  In many scheduler configurations,
this will result in an IPI back to the sender that is likely to be
received at a time when the sender CPU is idle.  The intent is to stress
IPI reception during CPU busy-to-idle transitions.

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