Fix FTBFS due to LTO being more sensitive to file order

When a symbol is used from inline assembly, LTO compiling and linking
process becomes more picky with regard to the order of compile/linking
units.

Specifically, when FreeRTOS/Source/portable/GCC/ARM_CM3/port.c comes
before FreeRTOS/Source/tasks.c in find results, the build fails with
undefined reference to `pxCurrentTCB' error.

To workaround the issue, do the same as we already have for
vTaskSwitchContext.

Note: different order affects resulting binary (.text section) size:
39924 with the wrong order and 39884 with the correct.

Fixes #685.

workspace/TS100/Makefile

@@ -95,7 +95,9 @@ LINKER_FLAGS=-Wl,--gc-sections 		\
 			-Wl,--wrap=free \
 			-o$(OUT_HEXFILE).elf 	\
 			-Wl,-Map=$(OUT_HEXFILE).map \
-			-lm -Wl,--undefined=vTaskSwitchContext \
+			-lm \
+			-Wl,--undefined=vTaskSwitchContext \
+			-Wl,--undefined=pxCurrentTCB \
 			-Wl,--defsym=__FLASH_SIZE__=$(flash_size) \
 			-Wl,--defsym=__BOOTLDR_SIZE__=$(bootldr_size) \
 			--specs=nano.specs