scripts/gdb: fix 'lx-dmesg' on 32 bits arch

The type atomic_long_t can have size 4 or 8 bytes, depending on
CONFIG_64BIT; it's only content, the field 'counter', is either an
int or a s64 value.

Current code incorrectly uses the fixed size utils.read_u64() to
read the field 'counter' inside atomic_long_t.

On 32 bits architectures reading the last element 'tail_id' of the
struct prb_desc_ring:
	struct prb_desc_ring {
		...
		atomic_long_t tail_id;
	};
causes the utils.read_u64() to access outside the boundary of the
struct and the gdb command 'lx-dmesg' exits with error:
	Python Exception <class 'IndexError'>: index out of range
	Error occurred in Python: index out of range

Query the really used atomic_long_t counter type size.

Link: https://lore.kernel.org/r/20220617143758.137307-1-antonio.borneo@foss.st.com
Fixes: e6076831

 ("scripts/gdb: update for lockless printk ringbuffer")
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
[pmladek@suse.com: Query the really used atomic_long_t counter type size]
Tested-by: Antonio Borneo <antonio.borneo@foss.st.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220719122831.19890-1-pmladek@suse.com