kernel: add private scheduler APIs

These functions are a subset of proposed public APIs to clean up
several issues related to safely handling waking of threads.  They
have been made private as they interface may change, but their use
will simplify the reimplementation of the k_work functionality.

See: https://github.com/zephyrproject-rtos/zephyr/pull/29668



Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>