lib: s390x: smp: guarantee that boot CPU has index 0

Guarantee that the boot CPU has index 0. This simplifies the
implementation of tests that require multiple CPUs.

Also fix a small bug in the allocation of the cpus array.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Fixes: f77c0515 ("s390x: Add initial smp code")
Fixes: 52076a63 ("s390x: Consolidate sclp read info")
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>