x/mobile/gl: Fix macro definitions for iOS
x/mobile/gl: Fix macro definitions for iOS

Now `gomobile bind -target ios` tries to compile a lib with
`darwin/arm`, `darwin/arm64`, and `darwin/amd64`. In the last
case, a tag `ios` is always added. In `x/mobile/gl`, the last case
`darwin/arm64` was not considered well and causes a compile error.
This CL fixes this compile error.

Fixes golang/go#16067

Change-Id: I56be73f373d9985c067a07e5084fa6b9f1ef76a0
Reviewed-on: https://go-review.googlesource.com/24140
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile