powerpc/smp: Factor out assign_threads()

Factor out the for loop that assigns CPU numbers to threads of a core.
The function takes the next CPU number to use as input, and returns the
next available CPU number after the threads has been assigned.

This will allow a subsequent change to assign threads out of order.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231229120107.2281153-4-mpe@ellerman.id.au