[S390] zcore: Add prefix registers to dump header

With this patch the prefix registers of all online CPUs are stored in the
the zcore dump header. This allows dump analysis tools to access the register
information that is stored in the prefix pages without using the System.map.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>