cpu/fe310: don't call thread_yield when sched_active_thread is invalid

As the comment above cpu_switch_context_exit notes:

	sched_active_thread is not valid when cpu_switch_context_exit() is called.

Unfortunately, thread_yield(), which is called directly by
cpu_switch_context_exit(), uses sched_active_thread possibly resulting
in a null pointer dereference.

Solution: Trigger a software interrupt to perform a context switch and
let sched_run() determine the next valid thread from there.

cpu/fe310/cpu.c

@@ -334,8 +334,8 @@ void cpu_switch_context_exit(void)
     /* enable interrupts */
     irq_enable();
 
-    /* start the thread */
-    thread_yield();
+    /* start the thread by triggering a context switch */
+    thread_yield_higher();
     UNREACHABLE();
 }