runtime: don't try to interrupt other cores before they are started

The GC shouldn't try to interrupt other cores before they are started.
For example, it would be possible for the GC to run in a package
initializer (which is currently run on a single core). That would
suggest questionable program design, but it is something that should
work. So this commit makes sure the GC only tries to scan the stack of
other cores when those other cores have in fact started.

src/runtime/scheduler_cores.go

@@ -14,6 +14,9 @@ const hasParallelism = true
 
 var mainExited atomic.Uint32
 
+// True after the secondary cores have started.
+var secondaryCoresStarted bool
+
 // Which task is running on a given core (or nil if there is no task running on
 // the core).
 var cpuTasks [numCPU]*task.Task
@@ -141,6 +144,7 @@ func run() {
 
 		// After package initializers have finished, start all the other cores.
 		startSecondaryCores()
+		secondaryCoresStarted = true
 
 		// Run main.main.
 		callMain()