powerpc: remove cpu_online_cores_map function

This function builds the cores online map with on-stack cpumasks which
can cause high stack usage with large NR_CPUS.

It is not used in any performance sensitive paths, so instead just check
for first thread sibling.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211105035042.1398309-1-npiggin@gmail.com