misc/cgo/testplugin: pass GO_GCFLAGS

The noopt builder sets GO_GCFLAGS when building the standard library.
Set it when building plugins to ensure the -shared packages built for it
have the same inlining in the export data (and thus the same package
version).

Tested locally with GO_GCFLAGS="-N -l" ./all.bash

Fixes #17937

Change-Id: Id037cfbf4af744c05c47bdc58eea60a5dba69533
Reviewed-on: https://go-review.googlesource.com/62511


Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>