go/internal/gccgoimporter: test case for issue 30628

Test case for a panic/crash in gccgoimporter caused by incorrect gccgo
export data emission. Note that the *.gox file checked in contains the
correct export data; the main intent of the test is to make sure that
gccgo produces the right export data when run on the Go source.

Updates #30628

Change-Id: I29c0c17b81a43f92ff64fbfcdc58fdb46a5be370
Reviewed-on: https://go-review.googlesource.com/c/go/+/165739


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