kernel: userspace: manipulate _thread_idx_map on per-byte basis

The sys_bitfield_(clear/set)_bit() work on pointer size element.
However, _thread_idx_map[] is a byte array. On little endian
systems, the bitops should work fine. However, on big endian
systems, changing the lower bits may actually be manipulating
memory outside the array when CONFIG_MAX_THREAD_BYTES is not
multiple of 4. So modify the code to perform bit ops on
a per-byte basis.

Fixes zephyrproject-rtos#72430

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

kernel/userspace.c

@@ -277,8 +277,10 @@ static bool thread_idx_alloc(uintptr_t *tidx)
 		if (idx != 0) {
 			*tidx = base + (idx - 1);
 
-			sys_bitfield_clear_bit((mem_addr_t)_thread_idx_map,
-					       *tidx);
+			/* Clear the bit. We already know the array index,
+			 * and the bit to be cleared.
+			 */
+			_thread_idx_map[i] &= ~(BIT(idx - 1));
 
 			/* Clear permission from all objects */
 			k_object_wordlist_foreach(clear_perms_cb,
@@ -308,7 +310,8 @@ static void thread_idx_free(uintptr_t tidx)
 	/* To prevent leaked permission when index is recycled */
 	k_object_wordlist_foreach(clear_perms_cb, (void *)tidx);
 
-	sys_bitfield_set_bit((mem_addr_t)_thread_idx_map, tidx);
+	/* _thread_idx_map[] is of uint8_t */
+	_thread_idx_map[tidx / 8] |= BIT(tidx % 8);
 }
 
 static struct k_object *dynamic_object_create(enum k_objects otype, size_t align,