scripts/gdb: fix aarch64 userspace detection in get_current_task

At least recent gdb releases (seen with 14.2) return SP_EL0 as signed long
which lets the right-shift always return 0.

Link: https://lkml.kernel.org/r/dcd2fabc-9131-4b48-8419-6444e2d67454@siemens.com


Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>