misc/cgo/test: disable unhelpful GCC warning

GCC 9 warns about a change in the ABI of passing structs with bitfields,
but we don't care.

Fixes #50987

Change-Id: Ica658d04172a42a7be788f94d31a714bb8c4766f
Reviewed-on: https://go-review.googlesource.com/c/go/+/382956


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>