scripts/gdb: fix failing KGDB detection during probe

scripts/gdb: fix failing KGDB detection during probe

Patch series "scripts/gdb: Fixes for $lx_current and $lx_per_cpu".

This series fixes several bugs in the GDB scripts related to the
$lx_current and $lx_per_cpu functions.  The changes were tested with GDB
10, 11, 12, 13, and 14.

Patch 1 fixes false-negative results when probing for KGDB

Patch 2 fixes the $lx_per_cpu function, which is currently non-functional
in QEMU-GDB and KGDB.

Patch 3 fixes an additional bug in $lx_per_cpu that occurs with KGDB.

Patch 4 fixes the incorrect detection of the current CPU number in KGDB,
which silently breaks $lx_per_cpu and $lx_current.

This patch (of 4):

The KGDB probe function sometimes failed to detect KGDB for SMP machines
as it assumed that task 2 (kthreadd) is running on CPU 0, which is not
necessarily the case.  Now, the detection is agnostic to kthreadd's CPU.

Link: https://lkml.kernel.org/r/20240425153501.749966-1-mail@florommel.de
Link: https://lkml.kernel.org/r/20240425153501.749966-2-mail@florommel.de
Signed-off-by: Florian Rommel <mail@florommel.de>
Cc: Andrew Jones <ajones@ventanamicro.com>
Cc: Deepak Gupta <debug@rivosinc.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>