test: add a test for gccgo compiler bug of missing type descriptor

The gccgo compiler did not generate type descriptor for a pointer
to a type alias defined in another package, causing linking error.
The fix is CL 210787. This CL adds a test.

Updates #36085.

Change-Id: I3237c7fedb4d92fb2dc610ee2b88087f96dc2a1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/210858


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>