This permits the user to override the code generation flag when they
know better. This is always a good policy for all flags automatically
inserted by the build system.

Doing this now so that I can write a test for #20290.

Update #20290

Change-Id: I5c6708a277238d571b8d037993a5a59e2a442e98
Reviewed-on: https://go-review.googlesource.com/42952
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>