cmd/cgo: disable GCC 9 warnings triggered by cgo code

GCC 9 has started emitting warnings when taking the address of a field
in a packed struct may cause a misaligned pointer. We use packed
structs in cgo to ensure that our field layout matches the C
compiler's layout. Our pointers are always aligned, so disable the warning

Fixes #29962

Change-Id: I7e290a7cf694a2c2958529e340ebed9fcd62089c
Reviewed-on: https://go-review.googlesource.com/c/159859


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>