runtime: add Goroutines

R=rsc
CC=golang-dev
https://golang.org/cl/3508041

src/pkg/runtime/debug.go

@@ -26,6 +26,9 @@ func GOMAXPROCS(n int) int
 // Cgocalls returns the number of cgo calls made by the current process.
 func Cgocalls() int64
 
+// Goroutines returns the number of goroutines that currently exist.
+func Goroutines() int32
+
 type MemStatsType struct {
 	// General statistics.
 	// Not locked during update; approximate.

src/pkg/runtime/proc.c

@@ -1192,3 +1192,10 @@ runtime·mid(uint32 ret)
 	ret = m->id;
 	FLUSH(&ret);
 }
+
+void
+runtime·Goroutines(int32 ret)
+{
+	ret = runtime·sched.gcount;
+	FLUSH(&ret);
+}