cmd/cgo: ignore unrecognized GCC warning group pragmas
cmd/cgo: ignore unrecognized GCC warning group pragmas

CL 159859 causes build failure with old clang versions (3.4.1) on FreeBSD 10.3/10.4.

Update #29962
Update #27619

Change-Id: I78264ac5d8d17eeae89a982e89aac988eb22b286
Reviewed-on: https://go-review.googlesource.com/c/160777
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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