s390x/arch_dump: use proper note name and note size

In binutils/libbfd (bfd/elf.c) it is enforced that all s390
specific ELF notes like e.g. NT_S390_PREFIX or NT_S390_CTRS
have "LINUX" specified as note name and that the namesz is
6. Otherwise the notes are ignored.

QEMU currently uses "CORE" for these notes. Up to now this has
not been a real problem because the dump analysis tool "crash"
does handle that. But it will break all programs that use libbfd
for processing ELF notes.

So fix this and use "LINUX" for all s390 specific notes to comply
with libbfd. Also set the correct namesz.

Reported-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
