rtos/hwthread: fix threadid generation

Looks like 7f2d3e2925 introduced a regression by incorrectly assigning
threads. The title of the commit message says that the intention was to
"derive threadid from SMP index", this is not what happens, however.
Instead threadid is assigned based on an index of all examined targets
in an SMP group.

This introduces two logical errors.

*Error 1*

Here is the code that assigns threads to harts:

```
foreach_smp_target(head, target->smp_targets) {
  struct target *curr = head->target;

  if (!target_was_examined(curr))
    continue;

  threadid_t tid = threads_found + 1;
  hwthread_fill_thread(rtos, curr, threads_found, tid);
```

Now, imagine a situation when we have two targets: `target.A` and
`target.B`.  Let's assume that `target.A` is NOT examined (it could be
under reset, for example).  Then, according to the algorithm when
assigning thread identifiers `target.B` will be assigned tid of 1. The
respected inferior on GDB side will be called `Thread 1`.

Now, imagine that `target.A` activates and succefully examined - OpenOCD
will re-assign thread identifiers. And now on GDB side `Thread 1` will
represent the state of `target.A`. Which is incorrect.

*Error 2*

The reverse mapping between `threadid` and targets does not take the
state of targets into account.

```
static struct target *
hwthread_find_thread(struct target *target, threadid_t thread_id)
...
  threadid_t tid = 1;
  foreach_smp_target(head, target->smp_targets) {
    if (thread_id == tid)
      head->target;
    ++tid;
  }
```

So the constructed mapping is incorrect. Since in example above
`Thread 1` will get mapped to `target.A`.

*Solution:*

It seems that threadids should be assigned based on position of the
thread in an smp group disregarding the target state.

Change-Id: Ib93b7ed3bb03696afdf56a105b333e22b9ec69b5
Signed-off-by: Parshintsev Anatoly <anatoly.parshintsev@syntacore.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8471
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Mark Zhuang <mark.zhuang@spacemit.com>
Tested-by: jenkins
Reviewed-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>

src/rtos/hwthread.c

@@ -133,7 +133,7 @@ static int hwthread_update_threads(struct rtos *rtos)
 			if (!target_was_examined(curr))
 				continue;
 
-			threadid_t tid = threads_found + 1;
+			threadid_t tid = threadid_from_target(curr);
 			hwthread_fill_thread(rtos, curr, threads_found, tid);
 
 			/* find an interesting thread to set as current */