runtime: clean up scanning of Gs

Use a real type for Gs instead of scanning them conservatively.
Zero the schedlink pointer when it is dead.

Update #7820

LGTM=rsc
R=rsc, dvyukov
CC=golang-codereviews
https://golang.org/cl/89360043

src/pkg/runtime/malloc.h

@@ -631,6 +631,7 @@ BitVector	runtime·stackmapdata(StackMap *stackmap, int32 n);
 
 // defined in mgc0.go
 void	runtime·gc_m_ptr(Eface*);
+void	runtime·gc_g_ptr(Eface*);
 void	runtime·gc_itab_ptr(Eface*);
 
 void	runtime·memorydump(void);

src/pkg/runtime/mgc0.go

@@ -9,6 +9,11 @@ func gc_m_ptr(ret *interface{}) {
 	*ret = (*m)(nil)
 }
 
+// Called from C. Returns the Go type *g.
+func gc_g_ptr(ret *interface{}) {
+	*ret = (*g)(nil)
+}
+
 // Called from C. Returns the Go type *itab.
 func gc_itab_ptr(ret *interface{}) {
 	*ret = (*itab)(nil)

src/pkg/runtime/proc.c

@@ -687,6 +687,21 @@ runtime·allocm(P *p)
 	return mp;
 }
 
+static G*
+allocg(void)
+{
+	G *gp;
+	static Type *gtype;
+	
+	if(gtype == nil) {
+		Eface e;
+		runtime·gc_g_ptr(&e);
+		gtype = ((PtrType*)e.type)->elem;
+	}
+	gp = runtime·cnew(gtype);
+	return gp;
+}
+
 static M* lockextra(bool nilokay);
 static void unlockextra(M*);
 
@@ -1746,7 +1761,7 @@ runtime·malg(int32 stacksize)
 		runtime·throw("runtime: bad stack.h");
 	}
 
-	newg = runtime·malloc(sizeof(G));
+	newg = allocg();
 	if(stacksize >= 0) {
 		stacksize = runtime·round2(StackSystem + stacksize);
 		if(g == m->g0) {