go/types: add gccgo sizes information

This will need to be updated from time to time as new targets are
added to gccgo. But that is better than always returning nil.

Change-Id: I04b8c4d0f8efa38e2a148eb2e38b16b09f0351c3
Reviewed-on: https://go-review.googlesource.com/c/158844


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>