add portDONT_DISCARD to pxCurrentTCB (#479)
add portDONT_DISCARD to pxCurrentTCB (#479)

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'
FreeRTOS/FreeRTOS-Kernel,FreeRTOS/FreeRTOS-Kernel