go/internal/gccgoimporter: fix test when using gccgo before GCC 7
go/internal/gccgoimporter: fix test when using gccgo before GCC 7

In TestObjImporter skip tests that use type aliases when using a
version of gccgo before GCC 7, since that is when type aliases were
added.

Fixes #29006

Change-Id: I676bae9f023931cf95ac9b4d4de893fe8517af9b
Reviewed-on: https://go-review.googlesource.com/c/152078
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

cloudflare/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,josharian/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,josharian/go,akutz/go,nawawi/go,nawawi/go,josharian/go,golang/go,nawawi/go,akutz/go,golang/go,josharian/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,akutz/go,josharian/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,josharian/go,golang/go,nawawi/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,josharian/go,golang/go