sched: Improve AVR optimization of reschedule_timer()

Tweak the AVR register pressure optimization in reschedule_timer() to
optimize it further.  This improves the performance of AVR timers when
there are several pending timers.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sched.c

@@ -136,14 +136,18 @@ reschedule_timer(struct timer *t)
 
     // Find new timer position and update list
     timer_list = pos;
-    while (pos->next && sched_is_before(pos->next->waketime, minwaketime))
-        pos = pos->next;
-    t->next = pos->next;
-    pos->next = t;
-
+    struct timer *prev;
+    for (;;) {
+        prev = pos;
         if (CONFIG_MACH_AVR)
             // micro optimization for AVR - reduces register pressure
-        barrier();
+            asm("" : "+r"(prev) : : "memory");
+        pos = pos->next;
+        if (!pos || !sched_is_before(pos->waketime, minwaketime))
+            break;
+    }
+    t->next = pos;
+    prev->next = t;
     return timer_list;
 }