misc/cgo/testshared: correct test of gccgo version number

We still don't run the gccgo tests, because they don't run in module mode.
But now we at least get the version number check right.

Change-Id: Ifde4512c30605d1cb7e3a521f381a05c783549b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/362996


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>