kernel: add context pointer to thread->fn_abort

For compatibility layers like CMSIS where thread objects
are drawn from a pool, provide a context pointer to the
exited thread object so it may be freed.

This is somewhat obscure and has no supporting APIs or
overview documentation and should be considered a private
kernel feature. Applications should really be using
k_thread_join() instead.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>