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.
d37f8def88