cmd/tipgodoc: add appenginevm build tag

Fixes golang/go#9381.

Change-Id: Ied5fd00475dde2b781cedff398dc9ee8d7f642a4
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1801
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>