runtime: interpret type information during garbage collection

R=rsc, dvyukov, remyoudompheng, dave, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/6945069

src/pkg/runtime/malloc.h

@@ -489,5 +489,6 @@ enum
 
 // defined in mgc0.go
 void	runtime·gc_m_ptr(Eface*);
+void	runtime·gc_itab_ptr(Eface*);
 
 void	runtime·memorydump(void);

src/pkg/runtime/mgc0.go

@@ -8,3 +8,8 @@ package runtime
 func gc_m_ptr(ret *interface{}) {
 	*ret = (*m)(nil)
 }
+
+// Called from C. Returns the Go type *itab.
+func gc_itab_ptr(ret *interface{}) {
+	*ret = (*itab)(nil)
+}