scripts/gdb/symbols: determine KASLR offset on s390 during early boot

Using lx-symbols during s390 early boot fails with:

    Error occurred in Python: 'utf-8' codec can't decode byte 0xcb in position 0: invalid continuation byte

The reason is that s390 decompressor's startup_kernel() does not create
vmcoreinfo note, and sets vmcore_info to kernel's physical base.  This
confuses get_vmcore_s390().

Fix by handling this special case.  Extract vm_layout.kaslr_offset from
the kernel image in physical memory, which is placed there by the
decompressor using the __bootdata_preserved mechanism, and generate a
synthetic vmcoreinfo note from it.

Link: https://lkml.kernel.org/r/20250515155811.114392-4-iii@linux.ibm.com


Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>