6g: better genembedtramp fatal error,

   and don't put inaccessible private methods
   in signature.

R=ken
OCL=23138
CL=23140

src/cmd/6g/obj.c

@@ -537,7 +537,7 @@ genembedtramp(Type *t, Sig *b)
 		if(c == 1)
 			goto out;
 	}
-	fatal("genembedtramp");
+	fatal("genembedtramp %T.%s", t, b->name);
 
 out:
 	if(d == 0)

src/cmd/gc/subr.c

@@ -2489,6 +2489,8 @@ expand0(Type *t)
 	u = methtype(t);
 	if(u != T) {
 		for(f=u->method; f!=T; f=f->down) {
+			if(!exportname(f->sym->name) && strcmp(f->sym->package, package) != 0)
+				continue;
 			if(f->sym->uniq)
 				continue;
 			f->sym->uniq = 1;