iOS Building

This PR adds build scripts for building RetroArch iOS.

These scripts support building RetroArch for non-jailbroken and jailbroken phones.

More detailed information can be found in the [README](https://github.com/maddox/RetroArch/blob/ios-build-scripts/ios/README.md).

## Emulator Cores

The first thing this adds is some scripts for building emulator cores for iOS right from the RetroArch project. Each core has it's own build script (`script/cores/nestopia`), and you can build all of the cores by running `script/build_cores`.

The cores' repos are cloned down under `ios/tmp` and then built. Once built they are then moved into `ios/modules` so that they're included in the iOS app when built.

Right now the only cores being included are nestopia and snes9x-next. More to come.

## iOS App

There is also a build script that will build just the iOS app. `script/build_app`.

By default, the app will be built with codesigning enabled. But you can disable this when running the script.

The resulting binary will be found in `ios/build/Release-iphoneos`

### With Codesigning

This will build the iOS app, codesign everything that needs to, and package it into a distributable IPA.

`script/build_app`

### Without Codesigning

This will simply build the app into a .app binary.

`NOCODESIGNING=true script/build_app`


## Other scripts

### Build everything

You can build everything in one fell swoop using `script/build`

### Clean

You can remove all of the emulator core repos to start from scratch by running `script/clean`