[release-branch.go1.9] cmd/compile: make -asmhdr work with type aliases

For "type T = U" we were accidentally emitting a #define for "U__size"
instead of "T__size".

Updates #22877.
Fixes #25561.

Change-Id: I5ed6757d697753ed6d944077c16150759f6e1285
Reviewed-on: https://go-review.googlesource.com/80759


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 2f588ff0)
Reviewed-on: https://go-review.googlesource.com/118475


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>