s390/cpuinfo: show number of online CPUs within a package

Show number of online CPUs within a package (which is
the socket in case of s390). For what it worth, present
that value as "siblings" field - just like x86 does.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>