xtensa, kernel/sched: Move next switch_handle selection to the scheduler

The xtensa asm2 layer had a function to select the next switch handle
to return into following an exception.  There is no arch-specific code
there, it's just scheduler logic.  Move it to the scheduler where it
belongs.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/include/ksched.h

@@ -541,4 +541,10 @@ static inline int _is_thread_user(void)
 #endif
 }
 #endif /* CONFIG_USERSPACE */
+
+/**
+ * Returns the switch_handle of the next thread to run following an interrupt.
+ */
+void *_get_next_switch_handle(void *interrupted);
+
 #endif /* _ksched__h_ */