runtime: ensure time.Sleep(d) sleeps at least d

[eliasnaur]

Account for the sleep queue base time in the computation of the wakeup time.

Tested with the following program on pico2.

func main() {
go func() {
for i := range 60 {
const delay = 20 * time.Millisecond
before := time.Now()
time.Sleep(delay)
if d := time.Since(before); true || d < delay {
log.Println(i, "actual", d, "delay", delay)
}
}
}()
time.Sleep(500 * time.Millisecond)
log.Println("******** done sleeping ********")
select {}
}

Without this change, the program would print lines such as:

17 actual 15.494ms delay 20ms
18 actual 15.49ms delay 20ms
19 actual 15.585ms delay 20ms
20 actual 15.493ms delay 20ms
21 actual 15.494ms delay 20ms
22 actual 15.487ms delay 20ms
23 actual 15.498ms delay 20ms
******** done sleeping ********
24 actual 15.548ms delay 20ms
25 actual 20.011ms delay 20ms
26 actual 20.01ms delay 20ms
27 actual 20.011ms delay 20ms
28 actual 20.015ms delay 20ms

Note that while more than one sleeping goroutine is in the timer queue, the sleep duration is 5ms short.

[dgryski]

LGTM but should probably have a second set of eyes from @aykevl.