I want to know from where SAM3U will know that xPortSysTickHandler is the handler function for systick interrupt. I searched all the files in the demo project and did not find that this handler is used in any function to configure the systick IRQ.
If you are looking at an older demo (think so) then the interrupt handler will have been installed directly into the vector table. If you find the vector table definition, which will be in the start up files (you don’t say which compiler you are using), then in the slot for the SysTick interrupt you will find the name of the handling function.
If you are using a newer demo then the name of the function might be mapped to the CMSIS equivalent in FreeRTOSConfig.h, as described in point 1 on the following page: http://www.freertos.org/FAQHelp.html