[S390] cleanup sysinfo and add system z9 specific extensions.

With System z9 additional fields have been added to the output of the
store system information instruction. This patch adds the new model
information field and the alternate cpu capability fields to the
output of /proc/sysinfo. While we at it clean up the code as well.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>