runtime: add a test for asynchronous safe points

This adds a test of preempting a loop containing no synchronous safe
points for STW and stack scanning.

We couldn't add this test earlier because it requires scheduler, STW,
and stack scanning preemption to all be working.

For #10958, #24543.

Change-Id: I73292db78ca3d14aab11bdafd26d03986920ef0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/201777
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/export_test.go

@@ -42,6 +42,8 @@ var PhysHugePageSize = physHugePageSize
 
 var NetpollGenericInit = netpollGenericInit
 
+const PreemptMSupported = preemptMSupported
+
 type LFNode struct {
 	Next    uint64
 	Pushcnt uintptr