kernel/sched: Make z_ready_thread() safe vs. already-running threads

This is part of the scheduler API, and was always just a synchronized
wrapper around the internal ready_thread() function.  But where the
internal users seem to be careful not to call it on threads that are
not known to be already queued or running, the general users in the
IPC code seem to be less strict.

Add a simple test to detect the case where a thread is already
running.  Right now this just loops over the array of CPUs, so is O(N)
in the CPU count even though N is never more than four for us
currently.  But this is possible without modifying data structures.  A
more scalable way to do this if we ever need to run on very parallel
systems would be to use another state bit for RUNNING, or to keep a
backpointer in the thread struct to the CPU it's running on, etc...

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>