runtime: clear f, arg to avoid leak in timerproc

I have seen this cause leaks where not all objects in a sync.Pool
would be reclaimed during the sync package tests.
I found it while debugging the '0 of 100 finalized' failure we are
seeing on arm, but it seems not to be the root cause for that one.

LGTM=dave, dvyukov
R=golang-codereviews, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/64920044

src/pkg/runtime/time.goc

@@ -217,6 +217,14 @@ timerproc(void)
 			if(raceenabled)
 				runtime·raceacquire(t);
 			f(now, arg);
+
+			// clear f and arg to avoid leak while sleeping for next timer
+			f = nil;
+			USED(f);
+			arg.type = nil;
+			arg.data = nil;
+			USED(&arg);
+
 			runtime·lock(&timers);
 		}
 		if(delta < 0) {