cmd/compile: add support for //go:nointerface for -G=3

This is used within Google's internal code repo, so getting it working
is a pre-req for enabling -G=3 by default.

Change-Id: Icbc570948c852ca09cdb2a59f778140f620244b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/343429


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>