runtime: use persistentalloc instead of SysAlloc in netpoll

Especially important for Windows because it reserves VM
only in multiple of 64k.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10138043

src/pkg/runtime/netpoll.goc

@@ -338,7 +338,7 @@ allocPollDesc(void)
 			n = 1;
 		// Must be in non-GC memory because can be referenced
 		// only from epoll/kqueue internals.
-		pd = runtime·SysAlloc(n*sizeof(*pd));
+		pd = runtime·persistentalloc(n*sizeof(*pd), 0);
 		for(i = 0; i < n; i++) {
 			pd[i].link = pollcache.first;
 			pollcache.first = &pd[i];