rcu: create rcu_my_thread_group_empty() wrapper

Some RCU-lockdep splat repairs need to know whether they are running
in a single-threaded process.  Unfortunately, the thread_group_empty()
primitive is defined in sched.h, and can induce #include hell.  This
commit therefore introduces a rcu_my_thread_group_empty() wrapper that
is defined in rcupdate.c, thus avoiding the need to include sched.h
everywhere.

Signed-off-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>