add symbols needed by OpenOCD

this is a backport of
https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/188 which is part of
FreeRTOS V10.4.2

source/Middlewares/Third_Party/FreeRTOS/Source/tasks.c

@@ -378,6 +378,13 @@ PRIVILEGED_DATA static UBaseType_t uxTaskNumber 					= ( UBaseType_t ) 0U;
 PRIVILEGED_DATA static volatile TickType_t xNextTaskUnblockTime		= ( TickType_t ) 0U; /* Initialised to portMAX_DELAY before the scheduler starts. */
 PRIVILEGED_DATA static TaskHandle_t xIdleTaskHandle					= NULL;			/*< Holds the handle of the idle task.  The idle task is created automatically when the scheduler is started. */
 
+/* Improve support for OpenOCD. The kernel tracks Ready tasks via priority lists.
+ * For tracking the state of remote threads, OpenOCD uses uxTopUsedPriority
+ * to determine the number of priority lists to read back from the remote target. */
+const volatile UBaseType_t uxTopUsedPriority = configMAX_PRIORITIES - 1U;
+
+
+
 /* Context switches are held pending while the scheduler is suspended.  Also,
 interrupts must not manipulate the xStateListItem of a TCB, or any of the
 lists the xStateListItem can be referenced from, if the scheduler is suspended.
@@ -2093,6 +2100,10 @@ BaseType_t xReturn;
 	/* Prevent compiler warnings if INCLUDE_xTaskGetIdleTaskHandle is set to 0,
 	meaning xIdleTaskHandle is not used anywhere else. */
 	( void ) xIdleTaskHandle;
+
+	/* OpenOCD makes use of uxTopUsedPriority for thread debugging. Prevent uxTopUsedPriority
+	 * from getting optimized out as it is no longer used by the kernel. */
+	( void ) uxTopUsedPriority;
 }
 /*-----------------------------------------------------------*/