This commit adds a tools/rcu/rcu-updaters.sh script that uses bpftrace
to print a histogram of the RCU update-side primitives invoked during
the specified time interval, or until manually terminated if no interval
is specified.

Sample output on an idle laptop:

@counts[poll_state_synchronize_rcu]: 6
@counts[synchronize_srcu]: 13
@counts[call_rcu_tasks_trace]: 25
@counts[synchronize_rcu]: 54
@counts[kvfree_call_rcu]: 428
@counts[call_rcu]: 2134

Note that when run on a kernel missing one or more of the symbols, this
script will issue a diagnostic for each that is not found, but continue
normally for the rest of the functions.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>