rcu: make RCU_BOOST default on RT

Since it is no longer invoked from the softirq people run into OOM more
often if the priority of the RCU thread is too low. Making boosting
default on RT should help in those case and it can be switched off if
someone knows better.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>