runtime: burn cpu before calling yield in windows runtime.systime

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/117670043

src/pkg/runtime/os_windows.c

@@ -280,7 +280,7 @@ runtime·systime(KSYSTEM_TIME *timeaddr)
 	KSYSTEM_TIME t;
 	int32 i;
 
-	for(i = 0; i < 10000; i++) {
+	for(i = 1; i < 10000; i++) {
 		// these fields must be read in that order (see URL above)
 		t.High1Time = timeaddr->High1Time;
 		t.LowPart = timeaddr->LowPart;