Give each SMP group a unique number.

This helps e.g. if there are 8 cores, and cores 0--3 are in one SMP
group while 4--7 are in another group. (And there are 2 gdb instances
connected, one debugging the first group, and one the second.)

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I7b6c9382eadf964529105eaf0411a42d48768668
Reviewed-on: https://review.openocd.org/c/openocd/+/6979
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/target.h

@@ -200,7 +200,7 @@ struct target {
 	bool rtos_auto_detect;				/* A flag that indicates that the RTOS has been specified as "auto"
 										 * and must be detected when symbols are offered */
 	struct backoff_timer backoff;
-	int smp;							/* add some target attributes for smp support */
+	int smp;							/* Unique non-zero number for each SMP group */
 	struct list_head *smp_targets;		/* list all targets in this smp group/cluster
 										 * The head of the list is shared between the
 										 * cluster, thus here there is a pointer */