cmd/compile: emit itabs and itablinks

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>