runtime: implement NumCPU for -scheduler=threads

For the threads scheduler, it makes sense to have NumCPU available.
For all other schedulers, the number of available CPUs is practically
limited to one by the scheduler (even though the system might have more
CPUs).

src/internal/task/task_threads.go

@@ -38,6 +38,8 @@ type state struct {
 // Goroutine counter, starting at 0 for the main goroutine.
 var goroutineID uintptr
 
+var numCPU int32
+
 var mainTask Task
 
 // Queue of tasks (see QueueNext) that currently exist in the program.
@@ -53,7 +55,7 @@ func OnSystemStack() bool {
 // startup, before starting any other goroutines.
 func Init(sp uintptr) {
 	mainTask.state.stackTop = sp
-	tinygo_task_init(&mainTask, &mainTask.state.thread)
+	tinygo_task_init(&mainTask, &mainTask.state.thread, &numCPU)
 }
 
 // Return the task struct for the current thread.
@@ -259,7 +261,7 @@ func runtimePanic(msg string)
 // that the 't' parameter won't escape (because it will).
 //
 //go:linkname tinygo_task_init tinygo_task_init
-func tinygo_task_init(t *Task, thread *threadID)
+func tinygo_task_init(t *Task, thread *threadID, numCPU *int32)
 
 // Here same as for tinygo_task_init.
 //
@@ -273,3 +275,7 @@ func tinygo_task_send_gc_signal(threadID)
 
 //export tinygo_task_current
 func tinygo_task_current() unsafe.Pointer
+
+func NumCPU() int {
+	return int(numCPU)
+}