runtime: use persistentalloc instead of mallocgc for itab

Reduces heap size.

R=golang-dev, remyoudompheng, bradfitz
CC=golang-dev
https://golang.org/cl/10139043

src/pkg/runtime/iface.c

@@ -85,7 +85,7 @@ itab(InterfaceType *inter, Type *type, int32 canfail)
 	}
 
 	ni = inter->mhdr.len;
-	m = runtime·malloc(sizeof(*m) + ni*sizeof m->fun[0]);
+	m = runtime·persistentalloc(sizeof(*m) + ni*sizeof m->fun[0], 0);
 	m->inter = inter;
 	m->type = type;
 

src/pkg/runtime/runtime.h

@@ -418,6 +418,7 @@ struct	Func
 };
 
 // layout of Itab known to compilers
+// allocated in non-garbage-collected memory
 struct	Itab
 {
 	InterfaceType*	inter;