target/cortex_m: add SMP support for Cortex-M

Cortex-M support for SMP multicore targets.

This SMP implementation unlike older ones does not act
on the first halted target found. It polls targets
until a SMP group is finished and stores eventual
'halted' events instead of emitting them. As soon as polling
of a group is done, poll proceeds with SMP related tasks.
This approach improves detection of a reason why debug
stopped - a correct reason is detected for all targets,
not only for the first found.
Drawback: SMP target group should be defined in the same
order as the targets were defined.

Obsolete gdb 'J' packet/smp_gdb command core switching is not implemented,
use with rtos hwthread.

Only one core is resumed if debug_execution is requested.

Some ideas taken from Graham Sanderson's [4936]
and src/target/aarch64.c

Added error checking of armv7m_restore_context().

Change-Id: I60f5b79e74b624dc2b5835ff10e38ac2ccb23792
Link: [4936]: target/cortex_m: Add smp support for Cortex M | https://review.openocd.org/c/openocd/+/4936
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7239
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/target.h

@@ -193,6 +193,10 @@ struct target {
 	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 */
+	bool smp_halt_event_postponed;		/* Some SMP implementations (currently Cortex-M) stores
+										 * 'halted' events and emits them after all targets of
+										 * the SMP group has been polled */
+
 	/* the gdb service is there in case of smp, we have only one gdb server
 	 * for all smp target
 	 * the target attached to the gdb is changing dynamically by changing