s390: Implement dump-guest-memory support for target s390x

With this patch dump-guest-memory on s390 produces an ELF formatted,
crash-readable dump.
In order to implement this, the arch-specific part of dump-guest-memory
was added:
target-s390x/arch_dump.c contains the whole set of function for writing
Elf note sections of all types for s390x.

Signed-off-by: Ekaterina Tumanova <tumanova@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
[fixed indentation, use CamelCase, rename note_t to Note, use S390CPU]
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>