runtime: preempt a goroutine which calls a lot of short system calls

A goroutine should be preempted if it runs for 10ms without blocking.
We found that this doesn't work for goroutines which call short system calls.

For example, the next program can stuck for seconds without this fix:

$ cat main.go
package main

import (
	"runtime"
	"syscall"
)

func main() {
	runtime.GOMAXPROCS(1)
	c := make(chan int)
	go func() {
		c <- 1
		for {
			t := syscall.Timespec{
				Nsec: 300,
			}
			if true {
				syscall.Nanosleep(&t, nil)
			}
		}
	}()
	<-c
}

$ time go run main.go

real	0m8.796s
user	0m0.367s
sys	0m0.893s

Updates #10958

Change-Id: Id3be54d3779cc28bfc8b33fe578f13778f1ae2a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/170138
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/export_test.go

@@ -34,6 +34,8 @@ var Fastlog2 = fastlog2
 var Atoi = atoi
 var Atoi32 = atoi32
 
+var Nanotime = nanotime
+
 type LFNode struct {
 	Next    uint64
 	Pushcnt uintptr