cmd/compile: make duplicate anonymous interface output deterministic

Ranging through a map is non-deterministic and there can be duplicate
entries in the set (with the same name) which don't have identical
definitions in some cases.

Fixes #27013

Change-Id: I378c48bc359c10b25b9238e0c663b498455b19fd
Reviewed-on: https://go-review.googlesource.com/129515


Reviewed-by: Russ Cox <rsc@golang.org>