gn: iOS packaging script

gn: iOS packaging script

This is enough to run DM on my iPad.

I've tweaks DM so that it can run as built by both GN and GYP.
When we kill off GYP, all the dm_main() nonsense goes away.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-iOS-Clang-iPadMini4-GPU-GX6450-Arm7-Debug,Build-Mac-Clang-arm64-Debug-GN_iOS

Change-Id: I59176bc203ee3180618b94ac5f9d291e0ad20b62
Reviewed-on: https://skia-review.googlesource.com/7757
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Hal Canary <halcanary@google.com>