runtime: avoid race with forcegc helper

While we are here, give the gc helper a real function name
that will appear in stack traces.

LGTM=rlh
R=rlh
CC=dvyukov, golang-codereviews
https://golang.org/cl/133470043

src/pkg/runtime/proc.c

@@ -2846,9 +2846,12 @@ sysmon(void)
 		lastgc = runtime·atomicload64(&mstats.last_gc);
 		if(lastgc != 0 && unixnow - lastgc > forcegcperiod && runtime·atomicload(&runtime·forcegc.idle)) {
 			runtime·lock(&runtime·forcegc.lock);
+			if(runtime·forcegc.g != nil) {
+				// Goroutine may be started but has not initialized g yet.
 				runtime·forcegc.idle = 0;
 				runtime·forcegc.g->schedlink = nil;
 				injectglist(runtime·forcegc.g);
+			}
 			runtime·unlock(&runtime·forcegc.lock);
 		}
 

src/pkg/runtime/proc.go

@@ -31,7 +31,10 @@ var parkunlock_c byte
 
 // start forcegc helper goroutine
 func init() {
-	go func() {
+	go forcegchelper()
+}
+
+func forcegchelper() {
 	forcegc.g = getg()
 	forcegc.g.issystem = true
 	for {
@@ -47,7 +50,6 @@ func init() {
 		}
 		gogc(1)
 	}
-	}()
 }
 
 // Gosched yields the processor, allowing other goroutines to run.  It does not