sched: Properly account for timeslicing in tickless mode

When adding a new runnable thread in tickless mode, we need to detect
whether it will timeslice with the runnable thread and reset the
timer, otherwise it won't get any CPU time until the next interrupt
fires at some indeterminate time in the future.

This fixes the specific bug discussed in #7193, but the broader
problem of tickless and timeslicing interacting badly remains.  The
code as it exists needs some rework to avoid all the #ifdef mess.

Note that the patch also moves _ready_thread() from a ksched.h inline
to sched.c.

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