In order to keep consistency with target=android, this CL turns
off gomobile build support for target=ios for programs that don't
import golang.org/x/mobile/app.
Change-Id: I423b042144aecfdc127726d0b97733c4d6532a81
Reviewed-on: https://go-review.googlesource.com/11985
Reviewed-by: David Crawshaw <crawshaw@golang.org>