runtime: force testing calls of netpoll to run on system stack

Fixes #35053

Change-Id: I31853d434610880044c169e0c1e9732f97ff1bdb
Reviewed-on: https://go-review.googlesource.com/c/go/+/202444
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/runtime/export_test.go

@@ -35,7 +35,6 @@ var Atoi = atoi
 var Atoi32 = atoi32
 
 var Nanotime = nanotime
-var Netpoll = netpoll
 var NetpollBreak = netpollBreak
 var Usleep = usleep
 
@@ -54,6 +53,12 @@ func LFStackPop(head *uint64) *LFNode {
 	return (*LFNode)(unsafe.Pointer((*lfstack)(head).pop()))
 }
 
+func Netpoll(delta int64) {
+	systemstack(func() {
+		netpoll(delta)
+	})
+}
+
 func GCMask(x interface{}) (ret []byte) {
 	systemstack(func() {
 		ret = getgcmask(x)