riscv: Don't reschedule on back-to-back interrupts

In some cases the 'reschedule' code path is executed when the current
thread is the same as the next thread in the ready Q. If this happens,
the swap_return_value of the thread is ifalsely being reset to -EAGAIN.

This commit prevents the rescheduling code to run if the current thread
is the same as the thread in the ready Q.

Signed-off-by: Wolfgang Reißnegger <gnagflow@fb.com>