runtime: add zero field to rtype

The rtype struct is meant to be a copy of reflect.rtype.  The
zero field was added to reflect.rtype in 18495:6e50725ac753.

LGTM=rsc
R=khr, rsc
CC=golang-codereviews
https://golang.org/cl/93660045

src/pkg/runtime/type.go

@@ -26,6 +26,7 @@ type rtype struct {
 	string     *string
 	*uncommonType
 	ptrToThis *rtype
+	zero      unsafe.Pointer
 }
 
 type _method struct {