rtos_int_exit would store RA at an offset of 4 byte from the SP,
where the offset should be 0.
This caused rtos_int_exit to overwrite variables in bss.
The riscv vectors.S in riscv component contains the trap vector, which is responsible to
defer interrupts and examine if a task context switch is needed, this change cleans up
this code by hiding all freertos details behind on two functions rtos_it_enter/exit and
their implementations are placed in freertos riscv port files.
