rcutorture: Make torture.sh --do-rt use CONFIG_PREEMPT_RT

The torture.sh --do-rt command-line parameter is intended to mimic -rt
kernels.  Now that CONFIG_PREEMPT_RT is upstream, this commit makes this
mimicking more precise.

Note that testing of RCU priority boosting is disabled in favor
of forward-progress testing of RCU callbacks.  If it turns out to be
possible to make kernels built with CONFIG_PREEMPT_RT=y to tolerate
testing of both, both will be enabled.

[ paulmck: Apply Sebastian Siewior feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>