cmd/compile: emit itabs and itablinks

See #14874

This change tells the compiler to emit itab and itablink symbols in
situations where they could be useful; however the compiled code does
not actually make use of the new symbols yet.

Change-Id: I0db3e6ec0cb1f3b7cebd4c60229e4a48372fe586
Reviewed-on: https://go-review.googlesource.com/20888
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Michel Lespinasse <walken@google.com>

src/runtime/runtime2.go

@@ -577,6 +577,8 @@ type _func struct {
 
 // layout of Itab known to compilers
 // allocated in non-garbage-collected memory
+// Needs to be in sync with
+// ../cmd/compile/internal/gc/reflect.go:/^func.dumptypestructs.
 type itab struct {
 	inter  *interfacetype
 	_type  *_type