kernel: sched: optimize for Meta IRQs == coop prios

Combining Meta IRQs with cooperative threads requires extra care to
return to pre-empted cooperative threads when returning from a Meta IRQ.
This is only needed when there are cooperative threads that are not also
Meta IRQs. This PR saves some space & time when the number of Meta IRQs
is equal to the number of available cooperative threads.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>