scripts/gdb/symbols: determine KASLR offset on s390

scripts/gdb/symbols: determine KASLR offset on s390

Use QEMU's qemu.PhyMemMode [1] functionality to read vmcore from the
physical memory the same way the existing dump tooling does this.
Gracefully handle non-QEMU targets, early boot, and memory corruptions;
print a warning if such situation is detected.

[1] https://qemu-project.gitlab.io/qemu/system/gdb.html#examining-physical-memory

Link: https://lkml.kernel.org/r/20250303110437.79070-1-iii@linux.ibm.com
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andrew Donnellan <ajd@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>