add portDONT_DISCARD to pxCurrentTCB
This fixes link failures with LTO:
/tmp/ccJbaKaD.ltrans0.ltrans.o: in function `pxCurrentTCBConst2':
/root/project/FreeRTOS/portable/GCC/ARM_CM4F/port.c:249: undefined reference to `pxCurrentTCB'
/usr/lib/gcc/arm-none-eabi/11.2.0/../../../../arm-none-eabi/bin/ld: /tmp/ccJbaKaD.ltrans0.ltrans.o: in function `pxCurrentTCBConst':
/root/project/FreeRTOS/portable/GCC/ARM_CM4F/port.c:443: undefined reference to `pxCurrentTCB'
@@ -337,7 +337,7 @@ typedef tskTCB TCB_t;
/*lint -save -e956 A manual analysis and inspection has been used to determine
* which static variables must be declared volatile. */
-PRIVILEGED_DATA TCB_t * volatile pxCurrentTCB = NULL;
+portDONT_DISCARD PRIVILEGED_DATA TCB_t * volatile pxCurrentTCB = NULL;
/* Lists for ready and blocked tasks. --------------------
* xDelayedTaskList1 and xDelayedTaskList2 could be moved to function scope but
